A pseudorandom number generator reduced in size while maintaining high security is disclosed. The generator has a state storage unit of 2 blocks (n bits per block) and a buffer of plurality of blocks, and mixes their contents to obtain a random number sequence. The mixing is done by a nonlinear transformation unit for inputting the stored content of the buffer to output data of the same size as the input data, a first linear transformation unit for inputting the content of the state storage unit and the output of the nonlinear transformation unit to store an output into the state storage unit, and a second linear transformation unit for inputting the stored content of the buffer and the stored content of the state storage unit to store an output into the buffer. The mixed content of the state storage unit is output as a random number sequence.