diff --git a/packages/backend/src/core/MfmService.ts b/packages/backend/src/core/MfmService.ts
index 2095ebca98..99adad3018 100644
--- a/packages/backend/src/core/MfmService.ts
+++ b/packages/backend/src/core/MfmService.ts
@@ -179,7 +179,8 @@ export class MfmService {
 					break;
 				}
 
-				case 'ruby': {
+					// this is here only to catch upstream changes!
+				case 'ruby--': {
 					let ruby: [string, string][] = [];
 					for (const child of node.childNodes) {
 						if (child.nodeName === 'rp') {
@@ -310,16 +311,24 @@ export class MfmService {
 								continue;
 							}
 							if (child.nodeName === 'rt') {
-								text += '$[ruby $[group ';
+								// the only case in which we don't need a `$[group ]`
+								// is when both sides of the ruby are simple words
+								const needsGroup = nonRtNodes.length > 1 ||
+									/\s|\[|\]/.test(getText(nonRtNodes[0])) ||
+									/\s|\[|\]/.test(getText(child)) ;
+								text += '$[ruby ';
+								if (needsGroup) text += '$[group ';
 								appendChildren(nonRtNodes);
-								text += '] ';
+								if (needsGroup) text += ']';
+								text += ' ';
 								analyze(child);
-								text += '] ';
+								text += ']';
 								nonRtNodes = [];
 								continue;
 							}
 							nonRtNodes.push(child);
 						}
+						appendChildren(nonRtNodes);
 					}
 					break;
 				}
diff --git a/packages/backend/test/unit/MfmService.ts b/packages/backend/test/unit/MfmService.ts
index d263aafa0f..e54c006a4f 100644
--- a/packages/backend/test/unit/MfmService.ts
+++ b/packages/backend/test/unit/MfmService.ts
@@ -158,16 +158,16 @@ describe('MfmService', () => {
 		});
 
 		test('ruby with spaces', () => {
-			assert.deepStrictEqual(mfmService.fromHtml('<p>a <ruby>Miss key<rp>(</rp><rt>ミスキー</rt><rp>)</rp> b</ruby> c</p>'), 'a Miss key(ミスキー) b c');
-			assert.deepStrictEqual(mfmService.fromHtml('<p>a <ruby>Misskey<rp>(</rp><rt>ミス キー</rt><rp>)</rp> b</ruby> c</p>'), 'a Misskey(ミス キー) b c');
+			assert.deepStrictEqual(mfmService.fromHtml('<p>a <ruby>Miss key<rp>(</rp><rt>ミスキー</rt><rp>)</rp> b</ruby> c</p>'), 'a $[ruby $[group Miss key] ミスキー] b c');
+			assert.deepStrictEqual(mfmService.fromHtml('<p>a <ruby>Misskey<rp>(</rp><rt>ミス キー</rt><rp>)</rp> b</ruby> c</p>'), 'a $[ruby $[group Misskey] ミス キー] b c');
 			assert.deepStrictEqual(
 				mfmService.fromHtml('<p>a <ruby>Misskey<rp>(</rp><rt>ミスキー</rt><rp>)</rp>Misskey<rp>(</rp><rt>ミス キー</rt><rp>)</rp>Misskey<rp>(</rp><rt>ミスキー</rt><rp>)</rp></ruby> b</p>'),
-				'a Misskey(ミスキー)Misskey(ミス キー)Misskey(ミスキー) b'
+				'a $[ruby Misskey ミスキー]$[ruby $[group Misskey] ミス キー]$[ruby Misskey ミスキー] b'
 			);
 		});
 
 		test('ruby with other inline tags', () => {
-			assert.deepStrictEqual(mfmService.fromHtml('<p>a <ruby><strong>Misskey</strong><rp>(</rp><rt>ミスキー</rt><rp>)</rp> b</ruby> c</p>'), 'a **Misskey**(ミスキー) b c');
+			assert.deepStrictEqual(mfmService.fromHtml('<p>a <ruby><strong>Misskey</strong><rp>(</rp><rt>ミスキー</rt><rp>)</rp> b</ruby> c</p>'), 'a $[ruby **Misskey** ミスキー] b c');
 		});
 
 		test('mention', () => {
@@ -181,7 +181,7 @@ describe('MfmService', () => {
 		test('ruby', () => {
 			assert.deepStrictEqual(
 				mfmService.fromHtml('<ruby> <i>some</i> text <rp>(</rp><rt>ignore me</rt><rp>)</rp> and <rt>more</rt></ruby>'),
-				'$[ruby $[group  <i>some</i> text ] ignore me] $[ruby $[group  and ] more]'
+				'$[ruby $[group  <i>some</i> text ] ignore me]$[ruby $[group  and ] more]'
 			);
 		});
 	});