Cyclone separator for separating solid particles from particle laden gas having an outlet pipe assembly which includes a pair of co-axial outlet pipes radially spaced apart from each other to form an annular passage therebetween. A plurality of accelerating air supplying nozzles are provided around the outer pipe of the outlet pipe assembly. A blower is provided to supply air under pressure to the annular passage and the nozzles. The air under pressure is discharged from the nozzles along the outer surface of the outlet pipe assembly in the same direction as that of a spiral downward flow of particle laden gas thereby accelerating the spiral downward flow and disturbing the outer surface of the outlet pipe assembly to prevent formation of a boundary layer. The air under pressure is also discharged from the annular passage to blow down the outermost portion of a spiral upward gas flow in the outlet pipe assembly which includes a substantial part of particles contained in the spiral upward flow, so that only the central portion of the spiral upward gas flow containing a less amount of particles is exhausted through the inner pipe of the outlet pipe assembly.
