To make perform precise control even if the characteristics of a system to be controlled have variance and to enable the controller to be of a high-density mounting.

A hole sensor 14 is controlled so that a constant current flows to a constant-current circuit 28, and outputs an output signal corresponding to the angle of rotation of a meter 13 when the meter 13 rotates. An adder 22 adds together the output signal from the hole sensor 14 and an offset voltage held in a PID arithmetic circuit filter arithmetic circuit 25 through a DAC 26. An amplifier 23 amplifies the signal from the adder 22. An arithmetic circuit 25 is supplied with parameters recorded in a ROM 32 through a microcomputer 31 so that an aperture diameter 11c reaches a target value. A driver circuit 29 performs the impedance conversion of the supplied voltage value and the impedance is applied as a driving voltage to a meter coil 30.