To enhance the safety of cipher text by updating a stored content state by an internal state updating means every time encryption conversion is executed.

An F function 1 as an encryption conversion means receives the plaintext to be encrypted, an encryption key 5 and the internal state 6, executes the prescribed encryption conversion and produces an output 7. An internal state updating function 3 updates the internal state 6 every time the encryption conversion in an internal state storing means 2 for storing the internal state 6 and the F function 1 is executed, i.e., simultaneously with the encryption conversion. Even if the same plaintext and the cipher key are inputted, the output is changed by the updatable internal state according to such constitution and, therefore, the safety of a round function which can be an effective defense method against the differential attack based on the analysis of the input and output functions of the round function is enhanced. The device may be composed of the round functions of a smaller number of stages when the safety of about the same degree is taken into consideration.