An air cleaner unit for an internal combustion engine includes a casing and a filter element disposed in the casing. The casing includes a first case member having a first opening and a first sealing surface on a periphery of the first opening and a second case member having a second opening and a second sealing surface on a periphery of the second opening. The first case member and the second case member are coupled such that the first and second openings are opposed in a substantially horizontal direction of a vehicle. The filter element has a peripheral portion that is interposed between the first sealing surface and the second sealing surface. The first case member includes a side wall and a projection that projects from an inner surface of the side wall. The filter element is held in a substantially vertical position in the casing through the projection.