PURPOSE: To obtain information on displacement as basic information capable of obtaining grinding resistance and the like by providing each displacement sensor capable of detecting displacement from the axial core of a spindle.

CONSTITUTION: A grinding wheel 11 is installed at the tip end of a grinding wheel shaft 1. The outer circumference of the end section of the grinding wheel shaft 1 at the grinding wheel side is provided with a first and a second coil 31 and 32 detecting displacement in the direction of a Y axis, and with a third and a fourth coil 51 and 52 detecting displacement in the direction of a X axis while these coils are disposed at equal intervals wherein these coils consist of a displacement detecting circuit. When a work piece is ground by the grinding wheel 11 installed at the tip end of the grinding wheel shaft 1, the displacement of the grinding wheel shaft 1 from its shaft core is detected based on the change of inductance of the first and second coils 31 and 32, and the third and fourth coils 51 and 52. This results in the acquisition of information on displacement as basic information capable of obtaining grinding resistance and the like.