To prevent out of focus without increasing the number of parts nor complicating constitution.

The diaphragm control mechanism outputs an opening signal to a diaphragm device equipped with a diaphragm aperture whose opening diameter is varied between a totally opened state and a totally closed state, and adjusts a voltage level of the opening signal to thereby control size of the opening diameter of the diaphragm aperture. The diaphragm control mechanism generates a closing signal for increasing/decreasing the voltage level corresponding to the size of the opening diameter of the diaphragm aperture decided based on a video signal, generates an opening signal at the voltage level to change the opening diameter of the diaphragm aperture to be in size corresponding to the voltage level of the closing signal based on the voltage level of the generated closing signal, and outputs the generated opening signal to the diaphragm device. If the voltage level of the closing signal is a voltage level corresponding to the opening diameter which causes a diffraction phenomenon in the diaphragm device, the diaphragm control mechanism generates an opening signal at the voltage level at which the opening diameter of the diaphragm aperture is made an opening diameter which does not cause the diffraction phenomenon.