This paper presents a knowledge extraction attack on the RAG system, specifically a novel attack technique called "IKEA." IKEA extracts information about an external knowledge base using natural queries instead of malicious inputs. Specifically, it leverages anchor concepts to generate natural queries and effectively "explores" the RAG system's knowledge through "Experience Reflection Sampling," which samples anchor concepts based on past query-response records, and "Trust Region Directed Mutation," which iteratively transforms anchor concepts under similarity constraints. Experimental results show that IKEA demonstrates higher extraction efficiency and success rates than existing attack techniques under various defense mechanisms. Furthermore, the proxy RAG system built using IKEA exhibits similar performance to the original RAG system, suggesting a risk of copyright infringement.