This paper proposes LightRetriever to address the efficiency issues in large-scale language model (LLM)-based text retrieval. Existing LLM-based retrieval requires significant computational effort for query encoding, leading to slowdowns and resource consumption. LightRetriever uses existing large-scale LLMs for document encoding, but dramatically improves speed by streamlining the query encoding process to the level of an embedding lookup. Experimental results using an A800 GPU demonstrate that query encoding speed is over 1,000x faster, overall search throughput is over 10x faster, and retrieval performance is maintained at an average of 95% across various tasks.