a first prism arranged on an optical path of a light beam;

first and second gratings arranged on the optical path of the light beam having passed through the first prism at positions different in a direction of grooves of either the first grating or the second grating;

a beam adjustment optical system arranged on the optical path of the light beam between the first prism and at least one grating of the first and second gratings, and configured to cause a first portion of the light beam to be incident on the first grating and to cause a second portion of the light beam to be incident on the second grating;

a first actuator configured to adjust an incident angle of the first portion on the first grating;

a second actuator configured to adjust an incident angle of the second portion on the second grating; and

a third actuator configured to adjust an energy ratio of the first portion and the second portion by adjusting either a position or posture of at least one optical element included in the beam adjustment optical system,

wherein the beam adjustment optical system includes a beam separation optical system arranged as overlapping a part of a cross section of the optical path of the light beam, configured to separate the second portion from the first portion by causing a part of the light beam to be transmitted therethrough, and configured to change the energy ratio of the first portion and the second portion by changing a position of at least one optical element included in the beam separation optical system by the third actuator,

wherein the beam separation optical system includes a plane-parallel substrate, and the third actuator moves the plane-parallel substrate in a direction intersecting with a surface perpendicular to the grooves, and

wherein the plane-parallel substrate includes an exit surface from which the second portion exits from an inside of the plane-parallel substrate toward the second grating, and an end surface facing the first portion and forming an acute angle with the exit surface.