PROBLEM TO BE SOLVED: To adjust brightness at a sufficient response speed while preventing hunting regardless of the set light quantity.SOLUTION: A diaphragm driving circuit 40 includes: a comparator 41 a proportional gain calculating circuit 42 and a rotation calculating circuit 43. The comparator 41 receives a set light quantity Ld and brightness signal components. The comparator 41 calculates a deviation &DgrL by subtracting the set light quantity Ld from the brightness signal components. The proportional gain calculating circuit 42 receives the set light quantity Ld. The proportional gain calculating circuit 42 calculates a proportional gain K by dividing the product of a reference gain and a reference light quantity by the set light quantity. The rotation calculating circuit 43 receives the deviation &DgrL and the proportional gain K. The rotation calculating circuit 43 calculates the rotation on the basis of the deviation &DgrL and the proportional gain K. The rotation calculating circuit 43 allows a diaphragm driving motor 21c to rotate by the calculated rotation.
