Abstract: An electric power supply system is provided for an automatic water discharge device for discharging water to flush a basin. An object detector detects a person before the basin. A brightness detector, which consumes less electric power than the object detector, detects a brightness surrounding the basin. An electric power is supplied to the object detector on a power supply cycle so as to intermittently render it operative and to the brightness detector on a brightness detect cycle. The brightness detect cycle is longer than the power supply cycle. After a set time period following disappearance of a person from the basin, the electric power is, on one hand, shut off from the object detector to render it inoperative and, on the other hand, supplied to the brightness detector on the brightness detect cycle immediately after the shut off of electric power to the object detector, to detect a brightness surrounding the basin.