diff --git a/CHANGELOG.md b/CHANGELOG.md
index a801bfc..b59bc9d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,8 @@
 ## 1.2.0 (2026年05月03日)
 * Debianでコンパイル出来る様に
 * マウス対応の追加
+* ファイルなしでも開ける様に
+* `:o`コマンドの追加
 
 ## 1.1.0 (2025年12月28日)
 * PgUp・PgDownで使ってページを動ける様に
diff --git a/main.cc b/main.cc
index 4e81459..d1645b9 100644
--- a/main.cc
+++ b/main.cc
@@ -39,13 +39,13 @@ OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 #include "src/hexeditor.hh"
 
 int main(int argc, char *argv[]) {
-  if (argc != 2) {
-    std::cerr << "usage: hexagon <filename>\n";
-    return 1;
-  }
+  std:: string filename;
+
+  if (argc > 1) filename = argv[1];
+  else filename = "";
 
   try {
-    HexEditor editor(argv[1]);
+    HexEditor editor(filename);
     editor.run();
   } catch (const std::exception &e) {
     std::cerr << "エラー： " << e.what() << "\n";
diff --git a/src/hexeditor.cc b/src/hexeditor.cc
index 44fc427..305d51f 100644
--- a/src/hexeditor.cc
+++ b/src/hexeditor.cc
@@ -80,7 +80,7 @@ const std::vector<HexEditor::FileSignature> HexEditor::signatures = {
 HexEditor::HexEditor(const std::string &filename)
   : curPos(0), dpOffset(0), bpr(16),
     statusMode(Status_Normal), modified(false),
-    running(true),
+    running(true), hasFile(!filename.empty()),
     lastSearchDir(Direction_Forward),
     headerLen(0), headerType("") {
   // SIGINT (CTRL + C)を無効に
@@ -123,39 +123,46 @@ HexEditor::HexEditor(const std::string &filename)
   refresh();
 
   // ファイルをバッファーに読み込む
-  std::ifstream file(filename, std::ios::binary);
-  if (!file) {
-    endwin();
-    throw std::runtime_error("ファイルを開くに失敗");
-  }
+  if (hasFile) {
+    std::ifstream file(filename, std::ios::binary);
+    if (!file) {
+      endwin();
+      throw std::runtime_error("ファイルを開くに失敗");
+    }
 
-  buf.assign((std::istreambuf_iterator<char>(file)), {});
-  file.close();
+    buf.assign((std::istreambuf_iterator<char>(file)), {});
+    file.close();
 
-  if (buf.empty()) {
-    endwin();
-    throw std::runtime_error("ファイルが空です");
-  }
+    if (buf.empty()) {
+      endwin();
+      throw std::runtime_error("ファイルが空です");
+    }
 
-  fname = filename;
+    fname = filename;
+    hasFile = true;
   
-  // ファイルヘッダー
-  for (const auto &sig : signatures) {
-    if (buf.size() >= sig.signature.size()) {
-      bool match = true;
-      for (size_t i = 0; i < sig.signature.size(); ++i) {
-        if (buf[i] != sig.signature[i]) {
-          match = false;
+    // ファイルヘッダー
+    for (const auto &sig : signatures) {
+      if (buf.size() >= sig.signature.size()) {
+        bool match = true;
+        for (size_t i = 0; i < sig.signature.size(); ++i) {
+          if (buf[i] != sig.signature[i]) {
+            match = false;
+            break;
+          }
+        }
+      
+        if (match) {
+          headerLen = sig.signature.size();
+          headerType = sig.type;
           break;
         }
       }
-      
-      if (match) {
-        headerLen = sig.signature.size();
-        headerType = sig.type;
-        break;
-      }
     }
+  } else {
+    fname = "[No File]";
+    buf.clear();
+    hasFile = false;
   }
 
   // ウィンドウを作成する
@@ -516,9 +523,13 @@ void HexEditor::handleCommand() {
   } else if (statusText == "noh") {
     lastSearch = "";
     isCommand = true;
+  } else if (statusText.rfind("o ", 0) == 0 || statusText.rfind("open ", 0) == 0) {
+    std::string path = statusText.substr(statusText.find(' ') + 1);
+    handleOpen(path);
+    isCommand = true;
   } else {
     statusMode = Status_Error;
-    statusText.clear();
+    statusText = "不正なコマンド。";
   }
 
   if (isCommand) {
@@ -538,6 +549,67 @@ void HexEditor::handleQuit(bool force) {
   }
 }
 
+void HexEditor::handleOpen(const std::string &path) {
+  if (path.empty()) {
+    statusMode = Status_Error;
+    statusText = "利用方法： :o <filename>";
+    render();
+    return;
+  }
+
+  std::ifstream file(path, std::ios::binary);
+  if (!file) {
+    statusMode = Status_Error;
+    statusText = "ファイルを開くに失敗： " + path;
+    render();
+    return;
+  }
+
+  std::vector<uint8_t> newbuf((std::istreambuf_iterator<char>(file)), {});
+  file.close();
+
+  if (newbuf.empty()) {
+    statusMode = Status_Error;
+    statusText = "ファイルは空です。";
+    render();
+    return;
+  }
+
+  buf = std::move(newbuf);
+  fname = path;
+  hasFile = true;
+  curPos = 0;
+  dpOffset = 0;
+  modified = false;
+  undoStack.clear();
+  redoStack.clear();
+  lastSearch.clear();
+  lastHexSearch.clear();
+  headerLen = 0;
+  headerType = "";
+
+  for (const auto &sig : signatures) {
+    if (buf.size() >= sig.signature.size()) {
+      bool match = true;
+      for (size_t i = 0; i < sig.signature.size(); ++i) {
+        if (buf[i] != sig.signature[i]) {
+          match = false;
+          break;
+        }
+      }
+      if (match) {
+        headerLen = sig.signature.size();
+        headerType = sig.type;
+        break;
+      }
+    }
+  }
+
+  statusMode = Status_Normal;
+  statusText = path + " を開きました。";
+  render();
+}
+
 void HexEditor::handleSave() {
   std::ofstream file(fname, std::ios::binary);
   if (file) {
@@ -862,6 +934,14 @@ void HexEditor::input() {
     if (statusMode != Status_Normal && statusMode != Status_Error) continue;
 
     ch = getch();
+
+    if (!hasFile && ch != ':' && ch != 'Z') {
+      statusMode = Status_Error;
+      statusText = "ファイルを読み込んでいません。「:o <file>」を御利用下さい。";
+      render();
+      return;
+    }
+
     if ((ch == 'j' || ch == KEY_DOWN) && curPos + bpr < buf.size()) {
       curPos += bpr; // 下
       if (statusMode == Status_Error) statusMode = Status_Normal;
diff --git a/src/hexeditor.hh b/src/hexeditor.hh
index b09e254..75fb1f8 100644
--- a/src/hexeditor.hh
+++ b/src/hexeditor.hh
@@ -79,6 +79,7 @@ class HexEditor {
     bool running;
     std::string lastSearch;
     bool isMouse = false;
+    bool hasFile = false;
 
     WINDOW *hexPanel;
     WINDOW *asciiPanel;
@@ -102,6 +103,7 @@ class HexEditor {
     void topbar();
     void statusbar();
     void handleCommand();
+    void handleOpen(const std::string &path);
     void handleSave();
     void handleQuit(bool force);
     void handleSearch();