To increase the number of set steps of a set member to be about twice without increasing the number of steps of a motor per unit time by actuating the set member two steps each at a set actuation, stopping it by returning it by the oddnumbered steps after stopping it at a position beyond a set position when the position is the odd-nubmer-th step from the set position.

A diaphragm device is constituted so that the largest diameter thereof is F4 and the smallest diameter thereof is F22. At the set actuation, the set member is actuated two steps each from the first set position by the normal rotation of the motor as it is shown in a Figure. In the case a predetermined set position is the odd-number-th step from the first set psosition, the set member is stopped at the position beyond the prescribed set position. Thereafter, the motor is reversely rotated and the set member is returned by the odd-numbered steps so that the set member is stopped at the prescribed set position. Thus, the number of set steps of the set member can be made almost twice without increasing the number of steps of the motor per unit time. Besides, the number of set steps can be increased and a set time can be shortened.