Abstract: Provided is an automatic gain control (AGC) apparatus of a cable modem. The apparatus includes: a gain controller for measuring a power level of digital input signals transmitted from outside, accumulating the measured values of the digital input signals for a predetermined period, acquiring an average size of the signals, and calculating an error value between the average signal size and a predetermined signal size; an amplitude control unit for receiving error values adjusted by the gain controller and controlling an amplitude of the signal; a low pass filter for receiving the signal outputted from the amplitude control unit and performing low pass filtering on the signal; and a signal size control unit for receiving the signal obtained from filtering of the low pass filter and controlling the signal size.