Abstract: In a four level FSK optical communication apparatus, signal light which is modulated in FSK with information is transmitted through an optical transmission line. The signal light is detected in an optical heteroclyne system by using local oscillation light, thereby producing intermediate frequency band signal which is then demodulated. A detection system comprises first and second delay detection circuits. In the first detection circuit, the intermediate frequency band signal is divided into first and second signals. The first signal is delayed by a delay time, and then mixed with a second signal which is not delayed to produce first delay detection signal. In the second delay detection circuit, one of the first signal delayed and the second signal not delayed is shifted in its phase by .pi./2. Then, signal having the phase shift of .pi./2 and signal having no phase shift are mixed to produce second delay detection signal. Thus, the demodulation of four level FSK modulation signal is performed.