Provided is an illuminance sensor in which a consumption current is independent of an illuminance level of incident light. Amplifiers (21 to 24) and a subtraction circuit (25) are driven by a constant current source (not shown). The subtraction circuit (25) outputs a differential voltage between output voltages of a photodetector element (15) and a photodetector element (16). Based on the differential voltage, a sample/hold circuit (30) performs sampling or holding of a voltage at one end of a capacitor (13). A switch (28) is ON when an output voltage of the subtraction circuit (25) starts to change, thereby fixing the voltage at the one end of the capacitor (13) to a reference voltage.