diff --git a/astro.config.mjs b/astro.config.mjs
index 2af9742..c163247 100644
--- a/astro.config.mjs
+++ b/astro.config.mjs
@@ -1,3 +1,5 @@
+import { readFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
 import { defineConfig } from 'astro/config';
 import react from '@astrojs/react';
 import tailwind from '@astrojs/tailwind';
@@ -9,9 +11,18 @@ import remarkLinkCard from 'remark-link-card-plus';
 import remarkBreaks from 'remark-breaks';
 import { remarkMermaidInjector } from './src/plugins/remark/remark-mermaid-injector.mjs';
 
+const SITE_URL = 'https://www.rowicy.com';
+// data URIはURL.canParse()を通り、かつdev/build/prodのどの環境・ポートでも
+// そのまま表示できるため、ホスト名に依存する絶対URLより確実(svgは537byteと小さい)。
+const LINK_CARD_FALLBACK_IMAGE_URL = `data:image/svg+xml;base64,${readFileSync(
+  fileURLToPath(
+    new URL('./public/images/link-card-fallback.svg', import.meta.url)
+  )
+).toString('base64')}`;
+
 // https://astro.build/config
 export default defineConfig({
-  site: 'https://www.rowicy.com',
+  site: SITE_URL,
   prefetch: {
     prefetchAll: true,
   },
@@ -35,7 +46,15 @@ export default defineConfig({
       remarkBreaks,
       [
         remarkLinkCard,
-        { cache: false, shortenUrl: true, thumbnailPosition: 'left' },
+        {
+          cache: false,
+          shortenUrl: true,
+          thumbnailPosition: 'left',
+          ogTransformer: og => {
+            if (og.imageUrl && URL.canParse(og.imageUrl)) return og;
+            return { ...og, imageUrl: LINK_CARD_FALLBACK_IMAGE_URL };
+          },
+        },
       ],
     ],
   },
diff --git a/public/images/link-card-fallback.svg b/public/images/link-card-fallback.svg
new file mode 100644
index 0000000..30c0001
--- /dev/null
+++ b/public/images/link-card-fallback.svg
@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" width="1200" height="630" viewBox="0 0 1200 630">
+  <rect width="1200" height="630" fill="#e5e7eb"/>
+  <text x="600" y="580" font-family="sans-serif" font-size="28" fill="#6b7280" text-anchor="middle" style="white-space: pre;" transform="matrix(1.843255, 0, 0, 1.969718, -498.188232, -568.284058)">No Image</text>
+  <g transform="matrix(0.05973, -0.000087, -0.000087, -0.05973, -2411.421875, -2687.429932)" stroke="none" data-darkreader-inline-stroke="" style="--darkreader-inline-stroke: none; transform-origin: 3009.48px 2984.53px;">
+    <path d="M3525 5783 c-189 -44 -352 -180 -464 -384 -30 -56 -82 -179 -100 -239 -17 -56 -57 -197 -86 -305 -15 -55 -36 -127 -47 -160 -10 -33 -23 -78 -28 -100 -4 -22 -20 -85 -35 -140 -15 -55 -30 -118 -35 -140 -4 -22 -15 -69 -25 -105 -18 -70 -21 -85 -55 -235 -29 -134 -43 -185 -50 -185 -14 0 -193 509 -259 740 -12 41 -26 89 -31 105 -5 17 -16 59 -25 95 -37 145 -90 291 -142 390 -30 58 -58 107 -62 110 -3 3 -18 21 -33 40 -164 218 -420 294 -635 188 -73 -35 -171 -129 -212 -202 -58 -103 -72 -163 -71 -298 1 -125 4 -147 41 -298 11 -47 24 -101 29 -120 4 -19 15 -62 25 -95 9 -33 21 -76 25 -95 5 -19 16 -60 25 -90 9 -30 22 -77 30 -105 32 -114 179 -545 204 -600 5 -11 19 -47 31 -80 43 -120 61 -168 79 -209 10 -22 29 -65 41 -95 l22 -54 -21 -26 c-11 -14 -46 -64 -77 -111 -31 -47 -65 -97 -75 -112 -11 -15 -19 -30 -19 -33 0 -4 -15 -30 -33 -58 -18 -29 -42 -70 -54 -92 -12 -22 -30 -56 -41 -76 -11 -19 -32 -71 -47 -115 -25 -71 -28 -96 -33 -253 -5 -191 1 -237 50 -363 105 -271 377 -586 633 -734 l86 -49 -7 -40 c-29 -183 -16 -344 36 -445 72 -141 197 -254 363 -329 84 -38 87 -39 216 -72 104 -27 128 -29 291 -29 97 0 190 4 206 9 16 4 65 16 109 26 94 21 227 81 305 138 120 87 225 229 275 371 28 80 49 232 42 296 l-6 51 57 36 c190 119 303 217 424 370 78 98 168 259 189 338 4 17 17 65 28 109 12 43 21 109 21 147 0 59 3 71 20 81 31 19 143 155 180 217 17 30 44 93 60 140 25 74 29 102 29 198 0 98 -4 122 -29 192 -68 192 -199 292 -417 316 -57 7 -63 10 -72 38 -6 16 -11 37 -11 45 0 8 -15 46 -34 86 -45 97 -129 181 -227 228 -112 55 -149 63 -297 63 -72 0 -132 3 -132 6 0 10 34 112 80 239 23 63 44 124 47 135 11 43 89 250 163 435 184 459 193 491 194 680 0 147 -6 174 -64 295 -101 208 -276 327 -475 324 -38 -1 -79 -3 -90 -6z m197 -2947 c136 -36 208 -122 251 -296 35 -146 -24 -320 -143 -420 -100 -84 -274 -81 -391 7 -104 79 -139 251 -95 480 21 110 29 128 78 163 101 74 194 94 300 66z m-1287 -134 c70 -22 138 -62 160 -95 8 -13 22 -48 31 -78 9 -30 19 -58 24 -64 13 -17 30 -104 37 -180 8 -104 -16 -182 -78 -249 -63 -68 -135 -98 -249 -103 -78 -4 -89 -2 -137 24 -104 56 -200 176 -223 279 -32 137 20 322 112 397 96 78 212 102 323 69z m1220 -1767 c-23 -275 -212 -460 -534 -519 -148 -28 -292 -18 -483 32 -58 15 -165 68 -221 109 -65 48 -132 139 -151 207 -19 64 -21 167 -4 213 14 38 20 40 62 14 17 -10 67 -31 111 -45 44 -15 94 -32 110 -37 17 -6 37 -12 45 -14 8 -1 58 -15 110 -30 122 -34 329 -45 430 -22 146 33 408 121 447 150 11 8 58 24 77 26 4 1 5 -37 1 -84z" style="fill: rgb(108, 114, 128);"/>
+  </g>
+</svg>
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