PURPOSE: To enable a proper laser driving current value to be monitored even if cathode potential of a laser diode changes by monitoring current flowing between terminals of a discharge resistor and by providing a subtraction circuit receiving the output from an adder circuit which receives the second output from the laser driving AC current output part and the second output from a laser driving DC current output part and the output from a monitoring circuit.

CONSTITUTION: The title item includes a monitoring circuit 5 for monitoring current flowing between terminals of a discharge resistor 3, and a subtraction circuit 7 receiving the output from an adder circuit 6 which receives the second output from a laser driving AC current output part 1 and the second output from a laser driving DC current output part 4 and the output from the monitoring circuit 5. An AC current Iac which becomes an optic output is output from the laser driving AC current output part 1 and a DC current Idc corresponding to the threshold current value Ith of light output is output from the laser driving DC current output 4. The total of each Iac and Idc is obtained by the adder circuit 6 and the value of current flowing through the resistor for discharging static electricity which is obtained by the monitoring circuit 5 is subtracted by the subtraction circuit 7.