To easily perform grinding work of a ground location in an internal peripheral surface between a ring-shaped workpiece and an obstacle, by grinding the ground location while pressing a rotary grinding tool to the ground location of the ring-shaped workpiece by an energizing means moved by a front/rear right/left movement drive means.

In a work mechanism part 60, a rotary bar 66 is rotated by driving an air motor 65, a longitudinal table 62 is before/behind moved by extension/contraction driving a longitudinal drive air cylinder 62A, also a right/ left table 64 is right/left moved by extension/contraction driving a lateral drive air cylinder 64A. In this way, in a condition that a blade part 66A in a tip end of the rotary bar 66 is pressed to a ground location 2C by energizing force of a spring 68, the ground location 2C of a workpiece 2 is moved over a front/ rear right/left total region, the ground location 2C is ground.