PURPOSE: To improve the minimum input sensitivity without reducing the maximum input sensitivity of a prescaler by providing an amplifier means amplifying an input signal and a limit means limiting the amplitude of an output signal of the amplification means so as not to exceed the maximum input sensitivity of the prescaler, and supplying the result to the prescaler.

CONSTITUTION: An amplitude limit circuit 19 is provided between an amplifier circuit 18 consisting of a differential amplifier 13 and of an emitter follower circuit 14 and a prescaler 15. With a small input signal amplitude supplied after the input signal is amplified by the amplifier circuit 18, the signal is supplied to the prescaler 15 without being subject to amplitude limit by the amplitude limit circuit 19. With a large input signal amplitude supplied, after the input signal is amplified by the amplifier means 18, the result is subject to amplitude limit in a way not exceeding the maximum input sensitivity of the prescaler 15 and fed to the prescaler 15. Thus, the excellent preamplifier circuit for the prescaler able to improve the minimum input sensitivity without reducing the maximum input sensitivity of the prescaler is obtained.