A variable-length block encryption circuit having a simplified structure and that reduces the load on a host computer. A plaintext memory and an encryption/decryption circuit for encrypting or decrypting original data in blocks. The original data is transferred to the plaintext memory from the exterior. The transferred original data is supplied to the encryption/decryption circuit and is subjected to encryption or decryption in blocks. The result data is substituted for the original data in the plaintext memory. When all transferred original data is substituted, the result data is externally output.