diff --git a/Sources/SwiftComplexityCore/Analysis/FunctionDetector.swift b/Sources/SwiftComplexityCore/Analysis/FunctionDetector.swift
index f12e160..a54f411 100644
--- a/Sources/SwiftComplexityCore/Analysis/FunctionDetector.swift
+++ b/Sources/SwiftComplexityCore/Analysis/FunctionDetector.swift
@@ -26,6 +26,10 @@ class FunctionDetector: SyntaxVisitor {
         return detectedFunctions
     }
 
+    override func visit(_ node: ProtocolDeclSyntax) -> SyntaxVisitorContinueKind {
+        return .skipChildren
+    }
+
     public override func visit(_ node: FunctionDeclSyntax) -> SyntaxVisitorContinueKind {
         let name = extractFunctionName(from: node)
         let signature = extractFunctionSignature(from: node)
diff --git a/Tests/SwiftComplexityCoreTests/Fixtures/protocol_decl.swift b/Tests/SwiftComplexityCoreTests/Fixtures/protocol_decl.swift
new file mode 100644
index 0000000..e7fc7e9
--- /dev/null
+++ b/Tests/SwiftComplexityCoreTests/Fixtures/protocol_decl.swift
@@ -0,0 +1,11 @@
+//
+//  protocol.swift
+//  swift-complexity
+//
+//  Created by Fumiya Tanaka on 2025/10/18.
+//
+
+protocol P {
+    var foo: String { get }
+    func bar()
+}
diff --git a/Tests/SwiftComplexityCoreTests/Fixtures/protocol_extension_decl.swift b/Tests/SwiftComplexityCoreTests/Fixtures/protocol_extension_decl.swift
new file mode 100644
index 0000000..95e50c8
--- /dev/null
+++ b/Tests/SwiftComplexityCoreTests/Fixtures/protocol_extension_decl.swift
@@ -0,0 +1,10 @@
+//
+//  protocol_extension_decl.swift
+//  swift-complexity
+//
+//  Created by Fumiya Tanaka on 2025/10/18.
+//
+
+extension P {
+    func foo() {}
+}
diff --git a/Tests/SwiftComplexityCoreTests/SwiftComplexityTests.swift b/Tests/SwiftComplexityCoreTests/SwiftComplexityTests.swift
index a87cb23..39c5077 100644
--- a/Tests/SwiftComplexityCoreTests/SwiftComplexityTests.swift
+++ b/Tests/SwiftComplexityCoreTests/SwiftComplexityTests.swift
@@ -251,6 +251,34 @@ struct FunctionDetectionTests {
         #expect(names.contains("method1"))
         #expect(names.contains("method2"))
     }
+
+    @Test("protocol")
+    func protocolDeclTests() async throws {
+        // Given
+        let code = try loadFixture("protocol_decl")
+        let sourceFile = Parser.parse(source: code)
+        let detector = FunctionDetector(viewMode: .sourceAccurate)
+
+        // When
+        let functions = detector.detectFunctions(in: sourceFile)
+
+        // Then
+        #expect(functions.count == 0)
+    }
+
+    @Test("protocol_extension")
+    func protocolExtensionDeclTests() async throws {
+        // Given
+        let code = try loadFixture("protocol_extension_decl")
+        let sourceFile = Parser.parse(source: code)
+        let detector = FunctionDetector(viewMode: .sourceAccurate)
+
+        // When
+        let functions = detector.detectFunctions(in: sourceFile)
+
+        // Then
+        #expect(functions.count == 1)
+    }
 }
 
 // MARK: - Output Formatter Tests