To provide a technology that can easily find even one LED device is abnormal in a light source configured by connecting and driving a plurality of LED devices in series.

A plurality of the LED devices are divided into groups of at least two pieces of LED devices or more mutually connected, and forward voltage drop measuring means is provided for measuring the summ of forward voltage drops of each LED device configuring each group. The forward voltage drop measuring means is provided with a voltage detector for taking up the sum of forward voltage drops of each LED device configuring each group, measuring means for measuring the forward voltage drop taken up at the voltage detector, means for comparing the measured forward voltage drop with a reference voltage, means for judging whether or not each measured voltage drop satisfies a predetermined criterion based on comparison results, and means for indicating judgment results.