diff --git a/src/overlay3d.ts b/src/overlay3d.ts
index 1784ccd..e77deb2 100644
--- a/src/overlay3d.ts
+++ b/src/overlay3d.ts
@@ -1,6 +1,7 @@
 import {
   circleToRing,
   closeRing,
+  densifyPolygonRing,
   WGS84Geodesic,
   Planar,
   type CircleState,
@@ -39,9 +40,10 @@ export function buildPolygonPath(
   geodesic: boolean,
   maxSegmentLengthMeters: number = 10000.0,
 ): google.maps.LatLngAltitudeLiteral[] {
-  const path = geodesic
-    ? WGS84Geodesic.createInterpolatePoints(points, maxSegmentLengthMeters)
-    : Planar.createInterpolatePoints(points);
+  // リングの閉じる辺（末尾 → 先頭）も他の辺と同じく補間する。隣り合う点の間だけを補間すると、
+  // 閉じる辺が直線の弦になり、塗りも輪郭もそこだけ直線になる。geodesic は大圏、geodesic=false は
+  // 緯度経度の線形補間（ポリラインの false と同じ線。線形補間は 10 km 刻みのまま）。
+  const path = densifyPolygonRing(points, geodesic, geodesic ? maxSegmentLengthMeters : 10000.0);
   return toLatLngAltitudePath(path);
 }
 
diff --git a/test/GeodesicPolygonPath.test.mjs b/test/GeodesicPolygonPath.test.mjs
new file mode 100644
index 0000000..00bf45a
--- /dev/null
+++ b/test/GeodesicPolygonPath.test.mjs
@@ -0,0 +1,55 @@
+import assert from 'node:assert/strict';
+import { mkdirSync, mkdtempSync, rmSync } from 'node:fs';
+import { after, test } from 'node:test';
+import { pathToFileURL } from 'node:url';
+import { resolve } from 'node:path';
+import { build } from 'tsup';
+import { createGeoPoint, Planar, WGS84Geodesic } from '@mapconductor/js-sdk-core';
+
+// The polygon path handed to google.maps.Polygon (geodesic: false, densified
+// by us). A geodesic ring must include its closing edge (last vertex -> first):
+// Google draws fill and outline from the same path, so a missing closing edge
+// leaves both of them a straight chord there.
+mkdirSync(new URL('../build/', import.meta.url), { recursive: true });
+const outDir = mkdtempSync(new URL('../build/geodesic-polygon-path-tests-', import.meta.url));
+after(() => rmSync(outDir, { recursive: true, force: true }));
+await build({
+    entry: { overlay3d: 'src/overlay3d.ts' },
+    outDir, format: ['esm'], platform: 'node', splitting: false,
+    dts: false, sourcemap: false, config: false, silent: true,
+    external: ['@mapconductor/js-sdk-core'],
+});
+const { buildPolygonPath } = await import(pathToFileURL(resolve(outDir, 'overlay3d.mjs')));
+
+const p = (latitude, longitude) => createGeoPoint({ latitude, longitude });
+const BOSTON = p(42.36, -71.06);
+const LONDON = p(51.5, -0.12);
+const ICELAND = p(64.1, -21.9);
+const TRIANGLE = [BOSTON, LONDON, ICELAND];
+
+test('a geodesic polygon path follows the great circle on its closing edge', () => {
+    const path = buildPolygonPath(TRIANGLE, true);
+    const expected = WGS84Geodesic.createInterpolatePoints([...TRIANGLE, BOSTON]).slice(0, -1);
+    assert.equal(path.length, expected.length);
+    path.forEach((q, i) => {
+        assert.ok(Math.abs(q.lat - expected[i].latitude) < 1e-9 && Math.abs(q.lng - expected[i].longitude) < 1e-9, `point ${i}`);
+    });
+    // Iceland -> Boston passes 55.6N at 53.2W (the straight chord passes 51.6N).
+    const mid = WGS84Geodesic.interpolate({ from: ICELAND, to: BOSTON, fraction: 0.5 });
+    const nearest = Math.min(...path.map((q) => Math.hypot(q.lat - mid.latitude, q.lng - mid.longitude)));
+    assert.ok(nearest < 0.1, `nearest ${nearest}`);
+});
+
+// geodesic=false も閉じる辺を緯度経度で線形補間する（ポリラインの false と同じ線）。
+test('a non-geodesic polygon path interpolates its closing edge linearly', () => {
+    const path = buildPolygonPath(TRIANGLE, false);
+    const expected = Planar.createInterpolatePoints([...TRIANGLE, BOSTON]).slice(0, -1);
+    assert.equal(path.length, expected.length);
+    path.forEach((q, i) => {
+        assert.ok(Math.abs(q.lat - expected[i].latitude) < 1e-9 && Math.abs(q.lng - expected[i].longitude) < 1e-9, `point ${i}`);
+    });
+    // Halfway along Iceland -> Boston the linear edge passes 53.2N (the Mercator chord 54.7N).
+    const mid = p((ICELAND.latitude + BOSTON.latitude) / 2, (ICELAND.longitude + BOSTON.longitude) / 2);
+    const nearest = Math.min(...path.map((q) => Math.hypot(q.lat - mid.latitude, q.lng - mid.longitude)));
+    assert.ok(nearest < 0.1, `nearest ${nearest}`);
+});