To easily separate fragments and liquid by separating liquid and the fragments by using a magnet separator.

A part or all of suspension is taken out of a machine and supplied to a decanter. The suspension contains liquid, substances having polishing work and fragments produced at the time of separation or lapping, and the fragments consist of semiconductor materials and metallic fragments themselves. Firstly, the materials having polishing work are separated from the remaining suspension in the decanter. A small amount of the segments and occasionally the materials having polishing work which are not separated actually exist in the liquid coming out of the decanter. This solid contained liquid is supplied to a magnet separator, and as the separator magnetically holds a solid part, liquid and the solid are separated from each other. The collected liquid and the materials having polishing work separated in the decanter are repeatedly used for separation of a semiconductor disc or lapping.