An image forming device employs a multi-laser beam system in which the light intensity of the second semiconductor laser is adjusted while simultaneously emitting laser beams from both the first and second semiconductor lasers. This configuration makes it possible to reduce the number of change-over circuits indispensable for line APC control of semiconductor lasers by one. Thus the maximum voltage stored in the second peak hold circuit is the voltage generated to light the first semiconductor laser and the second semiconductor laser simultaneously. The voltage generated to light the first semiconductor laser and the second semiconductor laser simultaneously is inevitably larger than the voltage generated to light only the semiconductor laser. For this reason, the second peak hold circuit does not require a separate change-over circuit to switch the circuit from the sampling state to hold state since the voltage stored in the second peak hold circuit is not updated when only the first semiconductor lights.