PURPOSE: To realize a highly accurate measurement which is not affected by the thermal expansion of peripheral units by taking measurements at a most recessed (projecting) part on the machining surface of a semiconductor wafer and at a most recessed (projecting) part on the upper surface of a chuck mechanism using two point in-process gauges and then determining the difference between both measurements.

CONSTITUTION: A rotary table 1 disposed below a spindle shaft equipped with a grinding wheel is provided with a chuck mechanism 2 and a two point in-process gauge 3 is disposed at a position not subjected to the effect of thermal expansion of the table 1. A measurement A is then taken by means of the gauge 3 between the most recessed part WA on the machining surface of a semiconductor wafer W and the upper surface of the chuck mechanism 2 with the semiconductor wafer W being sucked thereto. Similarly, a measurement B is taken between the most projecting part WB on the machining surface of the semiconductor wafer W and the upper surface of the chuck mechanism 2. Subsequently, the difference of measurements A-B is determined and the semiconductor water W is ground while sustaining parallelism between the upper surface of the chuck mechanism 2 and the lower surface of grinding wheel as much as possible.