This invention relates to a device at a dust filter for cleaning a gas flow from dust. The dust filter consists of an electrostatic precipitator or fabric filter (bag filter) and comprises a filter chamber (10) with gas inlet (12) and gas outlet (13). At the gas inlet a precollector (30) in the form of a louvre collector (31) is located for separating coarse dust, i.e. dust with a diameter exceeding 50-75 microns. In the order to obtain good collection at the louvre collector (31), the collected dust is evacuated by means of a partial gas flow, which is passed to a dust hopper (34) and a dust discharge unit (35). According to the invention, the dust discharge unit (35) comprises a cyclone collector (36), the inlet portion (37) of which is integrated with the dust hopper (34), and the outlet portion of which or central pipe (38) is formed with a passageway (39), the mouth (40) of which is located in the filter chamber downstream the louvre collector. The cyclone collector (36) is dimensioned so that the pressure drop over the louvre collector (31) amounting dimensioned so that the pressure drop over the louvre collector (31) amounting to between 0.0015 and 0.0030 atm is sufficient to drive the partial gas flow through the cyclone collector, which may be an axial cyclone or a tangential cyclone.