An extreme ultraviolet light generation device may comprise: a chamber provided with a through-hole; an introduction optical system configured to introduce the pulse laser beam into a first predetermined region inside the chamber through the through-hole; a target supply device configured to output the target toward the first predetermined region; a light source configured to irradiate a second predetermined region with light whose optical path in the second predetermined region has a transverse section that is longer along a direction perpendicular to a direction of movement of the target than along the direction of movement of the target, the second predetermined region including part of a trajectory of the target between the target supply device and the first predetermined region; and an optical sensor configured to detect light incident on the optical sensor from the second predetermined region to detect the target passing through the second predetermined region.