A line narrowing module includes an enclosure, a prism which is disposed in an internal space of the enclosure and through which light passes, a mounter which is disposed in the internal space and on which the prism is mounted, a fixing unit which is disposed in the internal space and fixes the prism to the mounter, and a light blocking member. The light blocking member is disposed in the internal space and blocks scattered light in the internal space, the scattered light produced from the light and traveling to the fixing unit.