In the encryption/decryption method, a random number sequence {ri} is generated on the basis of a given multiple-affine key system K and encryption is performed by an exclusive OR of the random number sequence {ri} with a plain text. Further, the multiple-affine key system K is automatically sequentially rewritten into a series of new multiple-affine key systems each time when the number of use times of the multiple-affine key system reaches a predetermined number and encryption of plain texts thereafter is continued while generating random numbers using the series of the rewritten multiple-affine key systems. Likewise, in decryption as well, since decryption is performed using a multiple-affine key system automatically rewritten each time when the number of use times reaches a predetermined number, a third party cannot reproduce the multiple-affine key system and therefore cannot decipher a cipher text.