PURPOSE: To increase the magnetic flux generated from a lens by operating a device so that a current more than given value is sent to the lens coil temporally and the current is reduced to the given value.

CONSTITUTION: A lens magnetic field is formed in a space between magnetic poles 2, 3 by excitation of a lens coil 1. A current IOL is fed from the B electric source to the lens coil 1 through a transistor 4, the value of the current is detected with a resistance 6 and the current is fed to a differential amplifier 9. The amplifier controls the exciting current IOL with the detecting current. A potentiometer P is connected to the amplifier to set the current IOL, and the amplifier is connected to an electric source 10 through a switch 11. Thus a matnetic flux ΦA is generated by setting a current IOL through operation of the potentiometer P, the switch 11 is closed and then opened, and a higher magnetic flux ΦB than the initial magnetic flux ΦA can be obtained at an idential exciting current value. The magnetic flux with high accuracy can be obtained to a specific exciting current.