To provide an air cleaner which can extend the life of a cleaner element and which can reduce the disturbance generated in an intake flow and to provide a measuring apparatus, for an air flow rate, by which an intake flow rate can be measured with high accuracy.

An intake flow which flows into from the entrance pipe 23 of an air cleaner 2 is divided into pieces by diffusion plates 28. The intake flow which is divided into pieces by the diffusion plates 28 is guided so as to hit a cleaner element 25 nearly uniformly. As a result, only a part of the cleaner element 25 is not contaminated extremely, and the life of the cleaner element 25 can be extended. The intake flow is diffused on the upstream side of the cleaner element 25 by the diffusion plates 28. As a result, air which passes the whole of the cleaner element 25 is guided to the inner wall surface of a cleaner case and it flows into an exit pipe 24 while its flow is being contracted. The intake flow which flows into the exit pipe 24 is guided to a flowmeter 3 in a state that its disturbance is reduced. Consequently, the measuring error of the flowmeter 3 can be reduced.