A portable, hand-held, combustion engine powered working machine comprises a tool unit (1) including a rotatable working tool (3), and a machine unit (2) including an internal combustion engine (11), an assembly (16) for supplying air and fuel to the engine, and an air cleaning system, which comprises a filter assembly (10) including at least two filters (84, 85), referred to as pre-filter (84) and main filter (85), for cleaning the air that shall be supplied to the engine. The pre-filter is accommodated in an upper filter chamber (86) between a top cover (83) and a pre-filter bottom (9), and the main filter is accommodated in a lower filter chamber under the pre-filter bottom, said lower filter chamber being defined by the underside of the pre-filter bottom, a second filter bottom (125) at a distance from the pre-filter bottom, and a closed, circumferential wall (120) between the pre-filter bottom and the second filter bottom, said closed, circumferential wall forming a downwards directed protrusion on the underside of the upper filter bottom.