PURPOSE: To improve the performance of a common mode rejection ratio CMRR at a high frequency band by detecting an in-phase component of an inputted signal so as to feed back the component to an input side thereby cancelling effectively the in-phase component of the signal.

CONSTITUTION: A voltage of an in-phase of a signal at an input terminal INA is outputted from an output terminal OUTA and a voltage of an in-phase of a signal at an input terminal INB is outputted from an output terminal OUTB. An in-phase component detection circuit 2 to detect the in-phase component of the signal inputted between input terminals consists of TRs Q7, Q8 and a constant current source I4, and the in-phase component detected by the circuit 2 is fed back to the input side through a feedback loop. TRs Q10, Q9 and Q11 form a negative feedback loop to feed back the in-phase component to the input side and act like cancelling the in-phase component of the output signal by feeding back the detected in-phase component. Thus, the deterioration in the common mode signal elimination ratio CMRR at a high frequency band is prevented and the frequency characteristic of the CMRR is considerably improved.