Included are a transmitter and a receiver caused to have a constant temperature. The transmitter includes: a semiconductor optical amplifier having a reflection mirror at a first end thereof; an optical waveguide having a first end coupled to a second end of the semiconductor optical amplifier; a wavelength demultiplexing filter having an input port coupled to a second end of the optical waveguide and a plurality of output ports having constant transmission wavelength intervals; reflection structures to reflect part of light output from the output ports, the reflection structures provided for the respective output ports of the wavelength demultiplexing filter; modulators to modulate light transmitted through the reflection structures, the modulators provided for the respective reflection structures; and a wavelength multiplexing filter having input ports coupled to the respective output ends of the modulators, transmission wavelength intervals of the input ports being identical to the transmission wavelength intervals of the wavelength demultiplexing filter, and having the output port coupled to a first end of an optical fiber. The receiver includes: a wavelength demultiplexing filter having an input port coupled to a second end of the optical fiber and a plurality of output ports having the same transmission wavelength intervals as the transmission wavelength intervals of the wavelength demultiplexing filter and an FSR obtained by multiplying the transmission wavelength interval by the number of the output ports; light receivers to receive light output from the output ports, the light receivers provided for the respective output ports of the wavelength demultiplexing filter; and a temperature controller to control the temperature of the wavelength demultiplexing filter.