A light quantity detecting apparatus is disclosed which includes: an integration detection section configured to output a signal that varies with integrated quantities of light received by a light-receiving device; a comparison section configured to compare the signal output by the integration detection section with a reference value in order to output a signal representing a result of the comparison; a dummy signal generation section configured to generate a signal equivalent to the comparison result signal output by the comparison section; a changeover section configured to change the output of the comparison section with the output of the dummy signal generation section in response to a control signal; and an integration signal generation section configured to generate a signal indicating an end of the integration of the quantities of received light in accordance with the signal forwarded via the changeover section.