To provide an optical receiving device and an automatic gain control method capable of removing distortion of frequency characteristics in a transmission system and performing AGC (Automatic Gain Control) control having good operability with low cost.

The optical receiving device 20 of an optical communication system 10 includes a light receiving element 31 for converting an optical signal to an electrical signal, amplifiers (32, 34, 37) for amplifying signals, a level scan circuit 41 for measuring an electric power value of a channel frequency, an AGC control circuit 43 for deciding gain control voltages Vc1 to Vc4, an equalizer 35 for correcting distortion of the frequency characteristics by the gain control voltages, and attenuators (33, 36). The gain control voltages are to be decided based on the electric power value measured by the level scan circuit 41. The AGC control circuit 43 determines the frequency characteristics of a transmission path after detecting a peak electric power value by channel frequency sweep. The automatic gain control method is to be performed by the level scan circuit 41, a peak electric power value detecting circuit 42, and the AGC control circuit 43.