To prevent penetration of water to an internal combustion engine as the increase of the loading space of an air cleaner and the increase of a cost are suppressed.

In an air cleaner having an air outlet 43 positioned on the lower side than the air inlet of an air cleaner 12, a partition wall 41 performing a damming up action for a flow of air is formed along the bottom part 12d, of a space 17 of a clean side positioned downstream from the air cleaner element, of an air cleaner case 12. When water penetrates in the space 17 on the clean side, the water only flows the bottom part 12d side of the space 17 by means of a specific gravity difference with air, a penetration moisture content is dammed up by the partitioning wall 41.