Cyclone separator for separating solid particles from particle laden gas having an outlet pipe assembly which includes a pair of co-axial outlet pipes spaced radially apart from each other to form an annular passage therebetween. The passage is connected through a so-called aspiration effect or jet pump effect type of suction device to ports located around the outlet pipe assembly. The outermost portion of a spiral upward flow of gas in the outlet pipe assembly which includes a substantial part of the particles contained in the upward gas flow in the outlet pipe assembly is drawn into the passage between the outlet pipes without disturbance of the upward flow and returned back and discharged through the ports around the outlet pipe assembly into a separating chamber to disturb the outer surface of the outlet pipe assembly thereby to prevent formation of a boundary layer on the outer surface of the outlet pipe assembly. The inner pipe of the outlet pipes is connected to an outlet chamber so that only the central portion of the spiral upward gas flow containing a less amount of particles is exhausted.