The hydrocyclone 1 comprises a body 2 comprising a cylindrical part 2a and a conical part 2b with its diameter gradually reduced from the lower part of the cylindrical part 2a; a treating object liquid introducing port 2c connected to the cylindrical part 2a; a foreign matter discharge port 2d formed by opening the lower part of the conical part 2b; an upper casing 3 temporarily storing clean water; and an outlet pipe 4 concentric with the body 2 and connected to the upper casing 3 disposed above the body 2. A cylindrical body 6 with a mesh-like peripheral face is inserted into the outlet pipe4 keeping a space from the inner face thereof through the upper casing 3, and a flow-out control part 7a is formed at the outlet in the upper casing 3 of the outlet pipe 4 by an annular body 7 surrounding a protruded part in the upper casing 3 of the cylindrical body 6.