To encrypt another piece of data during encrypting process of a certain piece of data, a memory 55 is provided in parallel with a feedback line 65 which feeds back data from an encrypting module 51using an encryption key K to a selector 54. When an interrupt IT for processing plaintext block data N.sub.1 is generated while plaintext block data M.sub.1 is processed, ciphertext block data C.sub.1 at timing of generation of the interrupt IT is made to be stored in a register 56. The ciphertext block data C.sub.1 stored in the memory 55 is made to be selected by the selector 54 at timing of completion of processing the plaintext block data N.sub.1, and processing the plaintext block data M.sub.1+1 is started.