A light amount measuring device, according to the present invention can include, a light-receiving sensor capable of outputting a voltage corresponding to an amount of incident light, an A/D converting portion capable of converting an input voltage into a digital value, a light amount calculating portion capable of calculating the amount of light incident on the light-receiving sensor on the basis of the digital value obtained by conversion by the A/D converting portion, a resistor connected between the A/D converting portion and the light-receiving sensor, a zener diode has one end connected between the A/D converting portion and the resistor, wherein the zener diode is capable of allowing a breakdown current flow from the one end to another end if an output voltage from the light-receiving sensor is not less than a predetermined value.