PURPOSE: To output at a rapid response speed by applying a change amount according to a voltage change amount at the collector side of a first transistor of a laser diode to a first and a second transistor bases superposing on a first and a second bias voltage during rise and fall of a pulse signal current.

CONSTITUTION: A means 20 to apply an amount of voltage change is provided which applies a change amount according to a change amount of a voltage at a collector side of a transistor Q1 of a laser diode 1 to bases of transistors Q1 and Q2 during build and fall of a pulse signal current supplied to the laser diode 1 superposing on bias voltages VB1 and VB2 which are output from bias voltage sources B1 and B2, respectively.