Abstract: A cyclone comprising a swirl cylinder, an inner tube disposed in the upper section of the swirl cylinder, an inlet duct arranged tangentially to the outer periphery of the swirl cylinder, a drop opening formed in the lower section of the swirl cylinder, and a straightening instrument disposed in a space directly below the inner tube. The straightening instrument includes a conical member coaxial with the inner tube and guide plates arranged along a conical surface at the outer periphery of the conical member. A lower section of each of the guide plates is curved to encounter the turn tangential velocity components of a circumferential spiral stream smoothly into axial velocities alone, and unnecessary vortices which are not required for separation of particles and which cause energy loss are eliminated.