1. An extreme ultraviolet light generation apparatus comprising:A. a chamber in which extreme ultraviolet light is generated by a target substance being irradiated with a laser beam to generate plasma from the target substance;
B. an optical unit including a laser beam introduction optical system for introducing the laser beam into the chamber;
C. a chamber reference member supporting the chamber and including a housing space in which the optical unit is housed;
D. a height positioning mechanism configured to position, at a predetermined installation position in the housing space, the optical unit to a predetermined installation height while contacting a first part of the optical unit; and
E. a movement mechanism configured to linearly move the optical unit in the horizontal direction in the housing space while keeping the optical unit at a guide height, and including a guide member provided with a guide surface that guides the optical unit to the installation position while contacting a second part of the optical unit, and a retraction part that is provided to the guide member and to which the second part is retracted from the guide surface when the optical unit reaches the installation position, the guide height of the optical unit while being guided to move toward the installation position by the guide surface being substantially equal to the installation height.