PURPOSE: To enhance the cipher strength by coupling a linear arithmetic part and a non-linear arithmetic part in each cipher element in series.

CONSTITUTION: In an LP encipherment cipher element 1 of a first stage, and in the LP ciphering cipher element 1 of a middle stage, a text T and a cipher sentence output C generated by the LP ciphering cipher element 1 of the pre- stage are inputted to a ciphering linear arithmetic part 2, respectively together with an L cipher key KL, and by this ciphering linear arithmetic part 2, an intermediate cipher sentence C' is generated. Subsequently, the intermediate cipher sentence C' generated by the enciphering linear arithmetic part 2 is inputted to a ciphering non-linear arithmetic part 3 together with a P cipher key KP, and by this ciphering non-linear arithmetic part 3, a cipher sentence C is generated, and this cipher sentence C is outputted to the ciphering cipher element 1 of the next stage or a decoding part.