PURPOSE: To automatically compensate for changes in the size of machining of a spherical member and eliminate the loss of time required for measuring the size of machining of the spherical member by constantly detecting the axial abrasion loss of a grinding wheel and to make the size of machining of the spherical member always timely and continuously stable.

CONSTITUTION: The axial abrasion loss of a grinding wheel 4 is detected by a detecting portion 51 which detects sensor light from a position sensor 12, and this information is sent to a control portion 52 to move the grinding wheel 4 so as to compensate for changes in the size of machining.