To provide the automatic power controller in which a leading of a transmission wave is not delayed to a degree of giving a destructive affect onto transmission information.

A traveling wave detection circuit 6 detects a traveling wave in transmission waves branched by a directional coupler 5, its detection signal is sampled by a sample-and-hold circuit 11, averaged by an integration device 12 and the result is fed to an adder 10. A sampling period of the sample-and- hold circuit 11 is increased by a timing generating circuit 14 more than that in a steady-state when a transmission request signal or a frequency revision request signal is given externally to the circuit 11 so that a gain of an automatic gain adjustment circuit whose gain is adjusted by an output signal of the adder 10 as a control voltage follows properly the leading of the transmission wave.