PURPOSE: To keep the modulation amplitude of a low frequency analog signal at a constant level, and to stabilize a modulation factor, by keeping the amplitude of the low frequency analog signal at the constant level, in a light modulation circuit in which the low frequency analog signal is superimposed on a digital main signal.

CONSTITUTION: In the light modulation circuit having a light emitting element driving circuit 1 that outputs a light signal on which the low frequency analog signal is superimposed for the amplitude of a light digital main signal to be inputted to a terminal 10, an LPF4 extracts the low frequency analog signal from the output of a light receiving element 3. The peak value of the signal is detected at a peak value detection circuit 6. After the detected voltage is compared with a reference voltage in a gain control signal generation circuit 7, the circuit 7 outputs a control signal to an amplifier 8 that is a gain variable amplifier, and applies a DC negative feedback on the circuit 1 so as to compress a difference between the above stated reference voltage and a peak value detecting voltage, and thereby, gain control is performed. Therefore, a new low frequency analog signal inputted from a terminal 11, after the amplitude being controlled, is superimposed on the main signal at the circuit 1. In this way, the modulation amplitude of the analog signal can be kept at the constant level.