An automatic gain control circuit includes: an amplification degree control section for calculating power of a digital value of a received signal, and for controlling an amplification degree of an amplifier of a receiving apparatus so that a difference between the power and predetermined reference power is zero; a received signal quality measuring section for measuring quality of the received signal; a reference power adjusting section for, upon finding that the measured quality of the received signal is below a fixed level, adjusting the reference power for a predetermined period of time; and a reference power updating section for causing the reference power adjusting section to continue to adjust the reference power until the quality of the received signal recovers to be at a predetermined recovery level or above, and for updating the reference power so that the reference power takes on an optimally adjusted value.