PURPOSE: To simplify the determination of the height of the finished surface by pressing a grinding tool at the set angle larger than the optimum grinding angle onto a workpiece and fixing the grinding tool at the optimum angle by a lock cylinder and a stopper and advancing the tool by the differential distance between the set angle and the optimum angle.

CONSTITUTION: When a grindstone 16 collides with a workpiece 4 and continues to feed it in Z direction, and the angle formed between the working surface and the workpiece 4 becomes (β), a sensor 12 transmits a signal, and the feed in Z direction of an automatic grinding apparatus 2 is suspended, and a lock cylinder 7 is advanced, and when a link mechanism 10 contacts with a stopper 8, the working surface is locked at the optimum grinding angle (α). Therefore, a minute distance (δ) is secured between the working surface of the grindstone 16 and the workpiece 4, and the grindstone 16 starts revolution in the locked state, and when the correction of the height position is necessary, the working apparatus 2 is fed further in Z direction by the amount corresponding to the correction value (δ), and grinding feed in Y direction is performed, and then the grindstone 16 starts the grinding work for the prescribed grinding position 17 on the workpiece 4. Thus, positioning is carried-out correctly and easily.