PURPOSE: To surely separate a heavy material component by prohibiting the short-circuit flow directly heading toward a material discharge pipe from an inflow port so as to prevent the integration of the material into the spacing between the housing and rotor drum of a hydrocyclone and within the rotor drum.

CONSTITUTION: This hydrocyclone is provided with a part tapered to a conical shape in the region of the discharge port of the heavy mud component existing at one end, the rotor 5 disposed at the other end and the central material discharge pipe 4 having the inlet opening 7 in the rotor region. The material discharge pipe 4 is extended toward the ejecting end of the cyclone for the heavy component and the inlet opening 7 of the material discharge pipe 4 is directed to an opposite side. The rotor 5 is composed to a drum shape and the end of the rotor existing in the inlet opening 7 of the central material discharge pipe 4 is formed to cover the inlet opening 7 by opening the front surface. Consequently, the sepn. of the heavy material component is surely executed with high efficiency.