PURPOSE: To obtain random numbers momentarily with a simple constitution, by eliminating DC and harmonics from a minute voltage of noise source and shifting the output via a voltage comparator to the post-stages sequentially in synchronizing with a clock signal at a serial/parallel conversion shift register.

CONSTITUTION: A minute voltage S1 produced at a noise source 1 is inputted to an amplifier 2 having a characteristic cutting of DC and harmonics, and an output signal S2 is converted into an output signal S3 of logical levels "0" and "1". A clock signal S4 is produced at an oscillator 4 and the signal S3 is sequentially shifted to the post-stage at a serial/parallel conversion shift register 5 in synchronizing with it. At readout, the address from a CPU6 is inputted to a register 5 as an output enable signal S5 from an address bus 7 via an address decoder 8, the content of the register 5 is outputted on a data bus 9 for the transfer to the CPU6 as random number. During readout, the content of register is stored through the inhibition of input of the signal S4 by a gate 10.