A dimming control system 1 includes: light source units each having a respective LED; and a control unit connected to the light source units via power supply lines. The control unit is connected to each of the light source units in series. The control unit includes: switch portions that control dimming of the LEDs by repeating an ON/OFF operation in a predetermined cycle; current detection circuits and voltage detection circuits that detect a current or a voltage supplied via the power supply lines; and a CPU that determines there is an abnormality in the power supply lines or the light source units when a value of the detected current or voltage exceeds a predetermined range while the switch portions are ON.