diff --git a/fog.js b/fog.js
new file mode 100644
index 0000000..7e9d0a6
--- /dev/null
+++ b/fog.js
@@ -0,0 +1,101 @@
+(() => {
+  'use strict';
+
+  document.addEventListener("DOMContentLoaded", () => {
+
+    const START_POS = [38.891, 139.824];
+    const FOG_RADIUS = 40; // 霧解除半径（m）
+
+    const titleEl = document.getElementById("title");
+    const startBtn = document.getElementById("start");
+    const stopBtn = document.getElementById("stop");
+
+    let map;
+    let playerMarker;
+    let watchId = null;
+
+    // 解除済みエリア
+    const revealedLayers = L.layerGroup();
+
+    function initMap() {
+      map = L.map("locationmap", { tap: true })
+        .setView(START_POS, 16);
+
+      L.tileLayer("https://{s}.tile.osm.org/{z}/{x}/{y}.png", {
+        attribution:
+          '&copy; <a href="http://osm.org/copyright">OpenStreetMap</a> contributors'
+      }).addTo(map);
+
+      revealedLayers.addTo(map);
+
+      playerMarker = L.marker(map.getCenter()).addTo(map);
+      playerMarker.bindPopup("STARTを押して探索開始").openPopup();
+
+      // PC検証用：クリックで移動
+      map.on("click", (e) => {
+        updateLocation(e.latlng);
+      });
+    }
+
+    function revealFog(latlng) {
+      const circle = L.circle(latlng, {
+        radius: FOG_RADIUS,
+        color: "#4fc3f7",
+        fillColor: "#4fc3f7",
+        fillOpacity: 0.4,
+        weight: 1
+      });
+
+      revealedLayers.addLayer(circle);
+    }
+
+    function updateLocation(latlng) {
+      map.panTo(latlng);
+      playerMarker.setLatLng(latlng);
+
+      revealFog(latlng);
+
+      playerMarker
+        .setPopupContent("探索中… 霧を解除しています")
+        .openPopup();
+    }
+
+    function onSuccess(pos) {
+      updateLocation(
+        L.latLng(pos.coords.latitude, pos.coords.longitude)
+      );
+    }
+
+    function onError() {
+      playerMarker
+        .setPopupContent("位置情報を取得できません")
+        .openPopup();
+    }
+
+    function startWatch() {
+      stopWatch();
+      watchId = navigator.geolocation.watchPosition(
+        onSuccess,
+        onError,
+        { maximumAge: 0, timeout: 3000, enableHighAccuracy: true }
+      );
+      titleEl.textContent = "探索中";
+    }
+
+    function stopWatch() {
+      if (watchId !== null) {
+        navigator.geolocation.clearWatch(watchId);
+        watchId = null;
+        titleEl.textContent = "STOP";
+        playerMarker
+          .setPopupContent("探索を停止しました")
+          .openPopup();
+      }
+    }
+
+    startBtn.addEventListener("click", startWatch);
+    stopBtn.addEventListener("click", stopWatch);
+
+    initMap();
+  });
+})();