PURPOSE: To effectively make the shape of a diaphragm aperture approximate to complete roundness without complicating the constitution of a device and increasing manufacturing cost.

CONSTITUTION: First/second sector groups 1 and 2 are independently driven to open/close by different stepping motors 8 and 8'. Supporting shafts 1a and 2a planted in the sector groups 1 and 2 respectively are fitted into fitting holes 4a and 5a provided by notching respective fixing rings 4 and 5. Driving shafts 1b and 2b planted in the sector groups 1 and 2 respectively are fitted into cam part 6a and 7a formed on respective rotary rings 6 and 7. Both sector groups 1 and 2 are rotated around the supporting shafts 1a and 2a respectively in such a manner the rotational forces of the rotary rings 6 and 7 are received by the driving shafts 1b and 2b, to form the diaphragm aperture of a prescribed shape. The corner part of the diaphragm aperture formed by the inner peripheral parts of the sectors adjacent each other of one sector group 2 is covered with the inner peripheral the sector of the other sector group 1, to obtain the aperture approximate to the complete roundness.