PURPOSE: To use even a single electrode laser diode whose frequency is not extended over a low frequency by applying CMI coding to an electric input signal so as to keep a mark rate of the electric input signal of any pattern string to 1/2.

CONSTITUTION: A power input signal 9 is converted into a CMI code signal by a CMI coding circuit 1, in which the signal 9 is converted into a CMI code signal whose mark rate is 1/2 even when the signal is a pattern signal of any mark rate. Then the signal is modulated into a CPFSK signal by a laser diode drive circuit 2 and a laser diode 3 and the result radiates an optical output signal 10. The signal 10 is fed to a front end circuit 4 as an optical input signal 11 and subject to heterodyne detection and the result is converted into an IF electric signal. The signal IF is amplified by an IF amplifier circuit 5 and subject to delay detection by a demodulation circuit 6 and restored to a base band signal. Then the signal is subject to identification reproduction by an identification circuit 7 and converted into a binary code signal at a CMI decoding circuit 8, and it is outputted as an electric output signal 12.