Lawn mower (1) comprising a load-bearing body (10), a motor (20) supported by the load-bearing body (10), a vertical axis cutting blade (30) connected to the motor (20), a bag (40) for containing the grass clippings applied to the back of the load-bearing body (10), a flange (50) provided with a series of vents (56) and applied to the body (10) so as to separate a central chamber (13) from the cutting blade (30). Said flange (50) is provided with a series of S-shaped radial fins (61) adapted to define a series of radial vents (56), said radial fins (61) and said radial vents (56) being equally distributed around the central rotation axis of the cutting blade (30).
