The object is to enable cipher communication even when a cipher key in a one-time pad cipher (Vernam cipher) is running short. A one-time pad encrypting part encrypts communication data by the one-time pad cipher by sequentially using part of a one-time pad cipher key stored in a one-time pad cipher key storage part, to generate encrypted data. A block-encrypting part encrypts communication data by a block cipher by using a block-cipher key stored in a block-cipher key storage part, to generate encrypted data. An encryption control part controls whether the communication data is to be encrypted by the one-time pad encrypting part, or by the block-encrypting part, depending on a remaining bit count of the one-time pad cipher key stored in the one-time pad cipher key storage part.