To efficiently perform inner side grinding with high accuracy without requiring any skill and intuition by arranging a contact type sensor adjacent to a grinding tool supporting base on a table for NC lathe.

An inner diameter of a work 9 is measured by a contact type sensor 5 mounted in a table 2 and comparison operation and grinding by a finishing grinding wheel 4 are repeated until the diameter reaches to a final grinding target diameter in the same way as a grinding step by a rough grinding wheel 3 when the measured value does not reach the final grinding target inner diameter. However, in the grinding by the finishing grinding stone 4, when X axial direction present value of the grinding wheel is not more than about 0.01 than movement target value of the grinding wheel, difference between the movement target value of the finishing grinding wheel 4 and X axial direction present value of the grindstone is calculated and the table 2 is moved to X axial direction as much as the difference amount.