Provided are an apparatus and method for measuring an antenna gain. The apparatus includes an antenna gain-to-noise-temperature (G/T) ratio calculator for calculating a G/T ratio using a reception noise power measured while an antenna is tracking the sun, a noise temperature measurer for measuring a noise temperature when a signal is transmitted or received through the antenna, and an antenna gain calculator for calculating an antenna gain using the calculated G/T ratio and the measured noise temperature. Thus, it is possible to easily measure the antenna gain of a ground station without a reference antenna or a satellite-mounted device capable of functioning as a reference antenna.