A lawn mower has a machine body, a grass bag detachably attached to the machine body for collecting therein grass clippings, a driving power source mounted on the machine body, a cutter blade driven by the driving power source for rotation to cut grass, and a cutter housing covering the cutter blade. The cutter blade has air lifting sections disposed at opposite end portions thereof for producing carrier air during rotation of the cutter blade. The cutter housing has a grass clipping delivery passage for directing grass clippings to the grass bag with the carrier air produced by the air lifting sections of the cutter blade, and an exhaust air return passage for returning only the carrier air, as exhaust air, from the grass bag to the cutter housing. The grass clipping delivery passage extends in a front-and-rear direction of the cutter housing. The exhaust air return passage is disposed adjacent to and extends substantially perpendicular to the grass clipping delivery passage. The exhaust air return passage has an exhaust air inlet disposed in a rear upper surface portion of the cutter housing and an exhaust air outlet facing a rear lower end of the cutter housing.