To provide an air cleaner that adopts thoroughgoing measures for waterproofing, and capable of securing and maintaining high efficiency.

An air cleaner is installed in a frame of a vehicle body down below a seat. An inlet duct 12 is arranged along the shape of the lower surface of the seat, an intake port 12a of the duct 12 is located under the front part of the seat, intaked air is introduced to a body of an air cleaner 11, and a cleaner cap 14 is provided at a position lower than the intake port 12a of the duct 12. A rib 14c projecting downward from the opening of an intake passage 14a integrally formed in the cleaner cap 14 is integrally provided, and a rib 15f extending in a height direction is integrally formed in the inner side wall of a cleaner case 15. The rib 15f is located approximately on the same plane as a rib 14c of the cleaner cap 14.