An extreme ultraviolet (EUV) light source apparatus in which a location or posture shift of an EUV collector mirror can be detected. The apparatus includes: a chamber; a target supply mechanism for supplying a target material into the chamber; a driver laser for irradiating the target material with a laser beam to generate plasma; a collector mirror having a first focal point and a second focal point, for reflecting light, which is generated at the first focal point, toward the second focal point; a splitter optical element provided in an optical path of the light reflected by the collector mirror, for splitting a part of the light reflected by the collector mirror; and an image sensor provided in an optical path of the light split by the splitter optical element, for detecting a profile of the light split by the splitter optical element.