PURPOSE: To obtain the device which is high in the secrecy of a ciphered digital data sequence and superior in stability by finding a numerical solution of a specific equation having irregular solutions on the basis of a given initial value, a coefficient, and a delay time and generating a random number sequence on the basis of the numeral solution.

CONSTITUTION: The coefficient value, and delay time of a nonlinear differential equation inputted from an input part 1 and the initial value of a nonlinear differential equation inputted from a counter 8 are inputted to a random number sequence generation part 2. As for image data inputted from an image input part 6, an image size calculation part 7 counts horizontal and vertical images of the image data. The random number sequence generation part 2 generates a random number sequence of the size found from those values. A matrix arithmetic part 4' performs matrix arithmetic for the random number sequence from the random number sequence generation part 2 to generate a random number matrix. The random number matrix from the matrix arithmetic part 4' and the image data from the image input part 6 are inputted to a ciphering processing part 5, which calculates exclusive OR between the random number matrix and image data to perform a ciphering processing.