A distortion compensating amplifier including a forward circuit and a feedback circuit and performing distortion compensation of a signal in the forward circuit by use of a gain of the feedback circuit, includes a detecting unit which detects that the gain of the forward circuit is changed, a calculating unit which calculates, when the detecting unit detects that the gain of the forward circuit is changed, the gain of the feedback circuit on the basis of an input signal to the forward circuit and an output signal from the forward circuit, and a feedback gain updating unit which updates the present gain of the feedback circuit with the gain calculated by the calculating unit.