PURPOSE: To enable measurement with simple setting and one measuring person in short time by inputting the detection signals from the 1st and 2nd sensors to the device, setting a reference plane, calculating the rate of distortion and calculating the rate of correction when said rate exceeds a specified range.

CONSTITUTION: A guide arm 12 is moved one turn around a seat surface 11a of a base 11 to be measured. The detection signal from a displacement sensor 15 is successively inputted to an A/D converter 18. On the other hand, the detection signals from position sensors 13, 14 are digitized by A/D converters 16, 17 and are inputted to a means 19 for deciding the measuring position. An instruction for taking in the data on the rate of displacement is outputted to a means 20 for taking in and deciding the rate of displacement for every attainment of a specified angle. The rate of displacement is inputted to a means 22 for setting a reference plane and calculating the rate of distortion by which the rate of distortion is calculated. The data on the rate of distortion is decided in a means 23 for deciding acceptance and reject and calculating the rate of correction and is plotted graphically by an X-Y plotter 25 via a means 24 for plotting the rate of distortion and the rate of correction.