PURPOSE: To efficiently remove the contaminant by arranging a hydrocyclone cylinder in the center of a vessel and providing a spiral passage device in the cylinder.

CONSTITUTION: The waste water is introduced into a settling pit 12 through a duct 10, and a contaminated fluid material 14 is separated from the pit 12. The contaminated waste water is then introduced into a separator in a hydrocyclone cylinder 66, the light contaminant is released from the opening 69 of the upper wall, and the heavy contaminant is released downward through an opening 71. A spiral duct is provided in the cylinder 66, and a tangential inlet duct 46 is communicated with an opening 72. The heavy solid contaminant and sludgy material are dropped into a stationary region 76. A hood 80 surrounding the cylinder 66 practically is positioned at the upper part. The purified water is received by a duct 94.