The light-amount adjusting system includes a light-amount adjusting unit that changes a size of an aperture through which light passes, an actuator that drives the light-amount adjusting unit, and a detector that detects an aperture state of the light-amount adjusting unit. The system further includes a deviation signal generator that generates a signal corresponding to a deviation between a target aperture state of the light-amount adjusting unit and an aperture state thereof detected by the detector, a driving signal generator that provides an offset to the signal corresponding to the deviation to generate a driving signal for driving the actuator, and a filter which perform a low-pass-filter processing on the driving signal. An output from the filter is used as the offset. The system can control the light-amount adjusting unit with high stability and accuracy at any position and in any driving direction thereof.