PURPOSE: To cope with various transmission bands with one kind of an optical transmitter and an optical receiver by using the one of a wide band type for a variable attenuation circuit and attachably and detachably mounting a variable slope circuit to the transmitter.

CONSTITUTION: When TV signals transmitted from a center by a coaxial cable are inputted to an input terminal, the level and frequency characteristics are adjusted by an ATT 10, an EQ 11, a BON 12, the variable attenuation circuit 2 and the variable slope circuit 3 and the signals are amplified by an amplifier 20, converted into optical TV signals by an LD and emitted to an optical fiber connected to an output terminal. Also, a part of the output TV signals of the amplifier 20 is outputted to a modulation degree monitoring terminal by a branching device 21. Then, the variable attenuation circuit 2 is the one of the wide band type and the one matched with the transmission frequency of a CATV system is made attachable and detachable for the variable slope circuit 3. Thus, just by changing the variable slope circuit 3 with the one corresponding to a desired band, the various bands are coped with.