To vary a current supplied to a light emitting element according to ambient temperature.

A controller 1 inputs the voltage of a temperature sensor TH in every cycle and outputs a 6-bit value corresponding to the voltage of the temperature sensor TH. The 6-bit output corresponds to the ambient temperature. Then an R-2R ladder type resistance circuit divides and outputs a source voltage VBB according to the 6-bit value outputted from the controller 1. A current control circuit composed of an operational amplifier 3, a Tr4, and a resistance R,3 controls the current flowing to the light emitting element 5 according to the output voltage of the R-2R ladder type resistance circuit. Consequently, the current flowing to the light emitting element 5 corresponds to the ambient temperature.