1. A pseudorandom number generator, comprising: a plurality of linear feedback shift registers which operate in synchronism with each other in response to a clock inputted in common to the linear feedback shift registers; a nonlinear function circuit for coupling output bit streams of the linear feedback shift registers with a nonlinear function and outputting a resulting bit stream; a shift register which operates in synchronism with the clock and inputs an output of the nonlinear function circuit as a serial input; setting means for setting initial values to the linear feedback shift registers and the shift register; and addition means for inputting a predetermined plurality of stored bits of the shift register and outputting a pseudorandom number string in accordance with input values thereto in synchronism with the clock.