Abstract: An optical wavelength stabilizing system including a laser diode for emitting signal light, a Peltier element for controlling a temperature of the laser diode, and a drive circuit for driving the Peltier element. The signal light is branched into two components by an optical coupler. In an optical path of one of the two components obtained by the optical coupler, there is inserted an optical filter having a portion whose transmittance continuously changes according to wavelength. The signal light component passed through the portion of the optical filter is detected by a photodetector. An output current from the photodetector is converted into a voltage, which is then input into a comparator. The comparator compares the voltage output from the photodetector and a reference voltage to input a voltage difference therebetween into the drive circuit.