PURPOSE: To attain the amplification with high gain by applying peak hold at first, demodulating the result at a filter in demodulating and amplifying a PAM signal and amplifying the demodulated signal.

CONSTITUTION: A signal made into a pulse with a minute pulse width and a minute amplitude to reduce the power with amplified by a buffer amplifier 11 and inputted to a peak hold circuit 14, the result is inputted to a pulse shaping device 13 to form a holding pulse. The peak value is held in the peak hold circuit 14, and the signal whose energy is increased is demodulated by a band- pass filter 15 and amplified and extracted by an output amplifier 16. In summing outputs of plural minute signal amplifiers with different sampling position, the noise is averaged and the S/N is improved.