To stably control light if brightness of a subject changes rapidly.

A light quantity adjusting device comprises a light source for continuously emitting illuminating light, an illuminating light converting means which intercepts the illuminating light for a prescribed period to convert the illuminating light to intermittent pulsed light, a difference calculating means for calculating a difference between an output value of brightness signals obtained from outside devices and a prescribed reference value, a pulsed light adjusting means for varying the prescribed period from the calculated difference to adjust a width of the pulsed light, a diaphragm for physically regulating the amount of the illuminating light, and a diaphragm operation means for operating the diaphragm when a value of the difference is greater than a first threshold value.