A signal amplifier circuit includes peak value holding circuit 11 receiving positive-phase input signal, peak value holding circuit 12 receiving negative-phase input signal, adder 13 adding the positive-phase input signal and output signal of peak value holding circuit 12, adder 14 adding the negative-phase input signal and output signal of the peak value holding circuit 11, non-inverting amplifier 15 amplifying output signal of adder 13, non-inverting amplifier 16 amplifying output signal of adder 14, peak value holding circuit 21 receiving positive-phase output signal of non-inverting amplifier 15, peak value holding circuit 22 receiving negative-phase output signal of non-inverting amplifier 16, adder 23 adding the positive-phase output signal and output signal of peak value holding circuit 22, adder 24 adding the negative-phase output signal and output signal of peak value holding circuit 21, and differential amplifier 29 amplifying difference between output signals of adders 23 and 24. Error in discrimination for small-amplitude signal embedded in tail is reduced.