An apparatus for detecting frequency response of audio signals of left and right channels. The audio signals of the left and right channels are alternately introduced at a predetermined periodic rate with intervals of a predetermined duration between times of signal introduction, the introduced audio signals are detected after being divided into a plurality of frequency band signals by band-pass filters, and one of the detected outputs is in turn relayed selectively. During the above described intervals of predetermined duration, an output end of detection device is grounded and the gain of the band-pass filters are made substantially equal to zero in synchronism with the timing of the grounding.