To measure grinding temperature accurately without affecting machining pressure.

A workpiece holding mechanism (10) which holds a workpiece (3) in a surface perpendicular to pressing direction, is provided with a cylindrical shaft section (12), a plate part (14) having an infrared ray transmitting part (16) through which infrared ray can be transmitted, and a lip seal (18) located on the back of the workpiece peripheral. The workpiece (3) is pressed against an abrasive pad (28) with a fluidic pressure supplied to a space (31) which is formed among the plate part, the lip seal and the workpiece. In such a condition that the workpiece is pressed against the abrasive pad, the workpiece is ground by moving it relatively to the abrasive pad, besides the grinding temperature is obtained by measuring the energy level of infrared ray radiated from the ground surface of the workpiece and the grinding surface of the abrasive pad, with a infrared radiation thermometer (30) located above the cylindrical shaft section.