To provide a common mode feedback circuit providing no lord on a differential input/output amplifier and evolving no error in its output voltage.

The common mode feedback circuit comprises: the first and second transistors M1, M2 converting the positive output 12 and negative output 13 of the differential input/output amplifier 11 into currents; a third transistor M3 converting a reference voltage 16 into a current; the fourth, fifth, and sixth transistors M4, M5, M6, each in diode connection, converting the currents converted by the first, second, and third transistors into voltages; an in-phase voltage component detection circuit 14 consisting of two resistors with the same value detecting the in-phase voltage components of the voltages converted by the fourth and fifth transistors; and a comparison circuit 15 comparing the in-phase voltages detected by the in-phase voltage component detection circuit to the voltage converted by the sixth transistor to feed back the resulting voltage to the differential input/output amplifier. The characteristics and the sizes of the first to sixth transistors are made to be the same.