PURPOSE: To supply a drive current independent of temperature by providing a temperature detection means and a temperature characteristic voltage generation means, generating a voltage having a temperature characteristic corresponding to that of the output voltage of an optoelectric transducer means, adding it to a voltage once, and applying it to a drive voltage generator means, thereby to compensate the temperature characteristic of a light branching means.

CONSTITUTION: By a temperature detection means 11 and a temperature characteristic compensation means 12, a voltage of a temperature characteristic corresponding to that of the output voltage of an optoelectric transducer 2 is generated. Then, this voltage is added to the output voltage of a constant voltage generator means 13. The added voltage has a temperature characteristic equal to that of the output voltage of the optoelectric transducer 2. Then, in a drive voltage generator means 15, the added voltage and the output voltage of the optoelectric transducer means are compared. The result is used to control the drive current of the pumping laser diode constituting the optoelectric transducer means, whereby the output level of the pumping laser diode can be controlled independently of the temperature characteristic of a light branch means 6.