PURPOSE: To contrive to largely prolong the useful life of a grinding wheel, by increasing the depth of cut of a truing tool in accordance with a reduction in the radius of a grinding wheel so that the grinding wheel can be trued by a required minimum depth of cut according to the degraded condition of the surface of the grinding wheel.

CONSTITUTION: When a command for truing the grinding wheel 12 is generated during a grinding cycle, the wheel 12 is trued by a diamond roll 22, in which case, the depth of cut of the roll 22 is determined in accordance with the radius of the wheel 12 by a controlling motor 26. Namely, the range of the radius of the wheel 12 is discriminated by a computer 40 on the basis of a signal from a grinding wheel radius detector 32, a set point of one of digital switches 35W38 is read, and a number of pulses corresponding to the set point thus read is supplied to a driving circuit 42 to true the grinding wheel 12. In this case, a feed screw shaft 13 is rotated, and the position of a wheel head 11 is corrected. Simultaneously, a detecting shaft 29 is also moved. By such truing, the radius of the grinding wheel 12 is reduced, and operation of a limit switch operated by a dog is changed over.