An information retrieval apparatus includes contents, an index data generating unit, a character frequency management data generating unit, a compressing/encrypting unit, a retrieval initializing unit, a full text retrieving unit, and a retrieval result displaying unit. The character frequency management data generating unit generates character frequency management data based on the contents. The compressing/encrypting unit compresses the contents and encrypts the character frequency management data. The retrieval initializing unit decrypts encrypted character frequency management data. The full text retrieving unit executes full text retrieval for compressed contents using the character frequency management data and index data when receiving a retrieval keyword. The retrieval result displaying unit decompresses a retrieval candidate selected from retrieval candidates and displays as a retrieval result.