PURPOSE: To provide a ciphering method and ciphering device which are structurally simple, permit easy packaging and lessen deterioration of reliability by subjecting plane text to cardinal number conversion in prescribed word length unit with random number as cardinal numbers.

CONSTITUTION: This ciphering device has a data input means 11 for inputting the plain text as binary numeral. The device has a data dividing means 12 for dividing the inputted data at every preset bit length and a bit length setting means 13 for setting the bit length as the word length according to a CPU. The device has a data storage means 14 which stores the data divided by this data dividing means 12 and a random number forming means 15 which randomly forms the integers of the size to be housed within the set bit length. Further, the device has a random number array storage means 16 which stores the random number array formed by this random number forming means 15 and a cardinal number converting means 17 which makes the data read out of the data storage means 14 correspondent successively to the random numbers and subjects respective data to cardinal number conversion with the corresponding random numbers as cardinal numbers.