A light amount measuring apparatus including a light amount measuring circuit and a power supply for supplying power to the light amount measuring circuit; wherein the light amount measuring circuit includes a light receiving device for receiving light and outputting an electric signal corresponding to light amount of the received light; a first switch for switching between electrical connection and disconnection between the light receiving device and the power supply; and a drive controller for controlling the first switch so that the first switch electrically connects the light receiving device to the power supply when the light receiving device is set to an activated state and electrically disconnects the light receiving device from the power supply when the light receiving device is set to a deactivated state.