Abstract: An air cleaner includes a case in which an inner cylindrical filter and an outer cylindrical filter are housed, the case having a case body that defines an opening end surface closed by a cover member. The case includes an air inlet provided to an outer circumference of the case body for supplying outside air and an exhaust outlet provided to a bottom of the case body at a downstream side in an air-flowing direction for discharging the air supplied through the air inlet and filtered through the inner cylindrical filter and the outer cylindrical filter. A mass flow rate sensor is provided in the exhaust outlet to measure the flow rate of the air. A flow straightening grid is provided at the upstream side of the mass flow rate sensor to straighten the flow of the air.