To transfer only solid from a classified material containing liquid and solid.

In order to transfer only solid from material to be classified containing liquid and solid to the downstream side, a spiral screw classifier with a horizontal screw has a main spiral screw inclined so that its downstream side is positioned at the upper side, and a horizontal screw arranged on the side of the main screw at the upstream side so as to be perpendicular to the main screw for re-transferring the solid which is not transferred to the downstream and remains when the solid contained in the material to be classified is transferred by the main screw.