An internal combustion engine (1) having a cylinder (5) having a rearwardly slanted axial line is provided with a plurality of cylindrical cyclone dust collection units (34) commonly connected to an upstream end of a filter dust collection unit (35). Each cyclone dust collection unit includes a main body (44, 45) internally defining a separation chamber (45C), and having an air inlet (31) communicating with outside and an air outlet (73) communicating with the filter dust collection unit, and the separation chamber is provided with a dust ejection hole (45D) directed tangentially in an obliquely forward direction. Thereby, dust ejected from the dust ejection hole is ejected away from the operator of the engine, and is prevented from depositing on the engine.