PURPOSE: To provide a frequency stabilization circuit for an optical multiplexer/ demultiplexer in which the transmission characteristic of an optical multiplexer the same as that of an optical demultiplexer is provided.

CONSTITUTION: An optical signal outputted from an absolute frequency stabilizing light source 35 is inputted to the optical multiplexer 31, and it is received by a first optical receiver 46 via an optical coupler 41. The signal is multiplied by the oscillation output of the oscillator 36 of the absolute frequency stabilizing light source 35 by a first multiplier 48, and is passed through a first filter, and an error signal is inputted to a first temperature adjusting circuit 32, then, temperature adjustment to generate the minimum error is performed. While, the optical signal of the absolute frequency stabilizing light source 35 sent out from the optical coupler 41 to a transmission line is received by a second optical receiver 57 via the optical demultiplexer 33, and multiplication by the oscillation output is performed by a second multiplier 59 similarly, and the error signal is inputted to a second temperature adjusting circuit 34 via a second filter 62, thereby, the temperature adjustment of the optical demultiplexer by the minimum error can be performed.