diff --git a/public/src/js/parsetja.js b/public/src/js/parsetja.js
index 8968aae..744781f 100644
--- a/public/src/js/parsetja.js
+++ b/public/src/js/parsetja.js
@@ -514,9 +514,11 @@
 			}else{
 				
 				var string = line.toUpperCase().split("")
-				
+
+				const abekobe = localStorage.getItem("abekobe") ?? "false";
+				const detarame = parseFloat(localStorage.getItem("detarame") ?? "0", 10);
+
 				for(let symbol of string){
-					const abekobe = localStorage.getItem("abekobe") ?? "false";
 
 					if (abekobe === "true") {
 						if (symbol === "1") {
@@ -534,6 +536,22 @@
 						}
 					}
 
+					if (detarame > 0) {
+						const randomValue = Math.random() * 100;
+						if (randomValue < detarame) {
+
+							const first = ["1", "2"];
+							const second = ["3", "4", "A", "B"];
+							if (first.includes(symbol)) {
+								const firstIndex = Math.floor(Math.random() * first.length);
+								symbol = first[firstIndex];
+							} else if (second.includes(symbol)) {
+								const secondIndex = Math.floor(Math.random() * second.length);
+								symbol = second[secondIndex];
+							}
+						}
+					}
+
 					var error = false
 					switch(symbol){
 						
diff --git a/public/src/js/songselect.js b/public/src/js/songselect.js
index 2696f89..9e32f5c 100644
--- a/public/src/js/songselect.js
+++ b/public/src/js/songselect.js
@@ -256,6 +256,12 @@ class SongSelect{
 					action: "abekobe",
 				});
 
+				this.songs.push({
+					title: "でたらめ",
+					skin: this.songSkin.customSettings,
+					action: "detarame",
+				});
+
 		this.songs.push({
 			title: strings.back,
 			skin: this.songSkin.back,
@@ -961,6 +967,20 @@ class SongSelect{
 								}
 								localStorage.setItem("abekobe", abekobe);
 							}, 100);
+						} else if (currentSong.action === "detarame") {
+							this.playSound("se_don");
+							setTimeout(() => {
+								let detarame = localStorage.getItem("detarame") ?? "0";
+								const input = prompt("でたらめになる確率をパーセントで入力してね！", detarame);
+								if (input === null) {
+									// キャンセル
+								} else if (input === "") {
+									input = "0";
+								} else {
+									detarame = input;
+								}
+								localStorage.setItem("detarame", detarame);
+							}, 100);
 						}
 		}
 		this.pointer(false)