There is provided an amplifier that achieves stable distortion compensation without using a pilot signal even when there is a difference in level in the base signal of a multicarrier signal. The amplifier is a feedforward amplifier for amplifying a multifrequency wideband signal without using a pilot signal, and includes an input monitoring section in a signal input stage and an output monitoring section in a signal output stage, whereby for fluctuations in level of the signal from the monitoring sections, when the signal level is decreased to cause a significant decrease in the level of adjacent channel leakage power (ACP) and when the signal level does not decrease to cause a slight increase in ACP level, the controller shifts a set ACP reception frequency to an opposite frequency of the multicarrier signal, thereby achieving stable distortion compensation even when there is a difference in level in the base signal of a multicarrier signal.