diff --git a/class-diagram.md b/class-diagram.md
new file mode 100644
index 00000000..c46f6acc
--- /dev/null
+++ b/class-diagram.md
@@ -0,0 +1,131 @@
+```mermaid
+classDiagram
+    class Renderable {
+        <<interface>>
+    }
+    class DeclarationBlock {
+    }
+    class RuleSet {
+        <<abstruct>>
+    }
+    class AtRuleSet {
+    }
+    class KeyframeSelector {
+    }
+    class AtRule {
+        <<interface>>
+    }
+    class Charset {
+    }
+    class Import {
+    }
+    class Selector {
+    }
+    class CSSNamespace {
+    }
+    class Settings {
+    }
+    class Rule {
+    }
+    class Parser {
+    }
+    class OutputFormatter {
+    }
+    class OutputFormat {
+    }
+    class OutputException {
+    }
+    class UnexpectedEOFException {
+    }
+    class SourceException {
+    }
+    class UnexpectedTokenException {
+    }
+    class ParserState {
+    }
+    class Anchor {
+    }
+    class CSSBlockList {
+        <<abstruct>>
+    }
+    class Document {
+    }
+    class CSSList {
+        <<abstruct>>
+    }
+    class KeyFrame {
+    }
+    class AtRuleBlockList {
+    }
+    class Color {
+    }
+    class URL {
+    }
+    class CalcRuleValueList {
+    }
+    class ValueList {
+        <<abstruct>>
+    }
+    class CalcFunction {
+    }
+    class LineName {
+    }
+    class Value {
+        <<abstruct>>
+    }
+    class Size {
+    }
+    class CSSString {
+    }
+    class PrimitiveValue {
+        <<abstruct>>
+    }
+    class CSSFunction {
+    }
+    class RuleValueList {
+    }
+    class Commentable {
+        <<interface>>
+    }
+    class Comment {
+    }
+
+    RuleSet <|-- DeclarationBlock: inheritance
+    Renderable <|.. RuleSet: realization
+    Commentable <|.. RuleSet: realization
+    RuleSet <|-- AtRuleSet: inheritance
+    AtRule <|.. AtRuleSet: realization
+    Selector <|-- KeyframeSelector: inheritance
+    Renderable <|-- AtRule: inheritance
+    Commentable <|-- AtRule: inheritance
+    AtRule <|.. Charset: realization
+    AtRule <|.. Import: realization
+    AtRule <|.. CSSNamespace: realization
+    Renderable <|.. Rule: realization
+    Commentable <|.. Rule: realization
+    SourceException <|-- OutputException: inheritance
+    UnexpectedTokenException <|-- UnexpectedEOFException: inheritance
+    Exception <|-- SourceException: inheritance
+    SourceException <|-- UnexpectedTokenException: inheritance
+    CSSList <|-- CSSBlockList: inheritance
+    CSSBlockList <|-- Document: inheritance
+    Renderable <|.. CSSList: realization
+    Commentable <|.. CSSList: realization
+    CSSList <|-- KeyFrame: inheritance
+    AtRule <|.. KeyFrame: realization
+    CSSBlockList <|-- AtRuleBlockList: inheritance
+    AtRule <|.. AtRuleBlockList: realization
+    CSSFunction <|-- Color: inheritance
+    PrimitiveValue <|-- URL: inheritance
+    RuleValueList <|-- CalcRuleValueList: inheritance
+    Value <|-- ValueList: inheritance
+    CSSFunction <|-- CalcFunction: inheritance
+    ValueList <|-- LineName: inheritance
+    Renderable <|.. Value: realization
+    PrimitiveValue <|-- Size: inheritance
+    PrimitiveValue <|-- CSSString: inheritance
+    Value <|-- PrimitiveValue: inheritance
+    ValueList <|-- CSSFunction: inheritance
+    ValueList <|-- RuleValueList: inheritance
+    Renderable <|.. Comment: realization
+```
diff --git a/composer.json b/composer.json
index ddc7eb13..4662b79a 100644
--- a/composer.json
+++ b/composer.json
@@ -33,7 +33,8 @@
         "phpstan/phpstan": "^1.11.11",
         "phpstan/phpstan-phpunit": "^1.4.0",
         "phpunit/phpunit": "^8.5.38",
-        "rector/rector": "^1.2.4"
+        "rector/rector": "^1.2.4",
+        "tasuku43/mermaid-class-diagram": "^0.1.0"
     },
     "suggest": {
         "ext-mbstring": "for parsing UTF-8 CSS"