PURPOSE: To enable measurement with simple setting and one measuring person in short time by setting a reference plane and calculating the rate of distortion with respect to the reference plane from the 1st sensor which detects the rotating angle of an arm and the 2nd sensor which detects the displacement of the seat surface and calculating the rate of correction.

CONSTITUTION: A guide arm 13 makes one turn around the 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 17. On the other hand the detection signal from a rotating angle sensor 14 is digitized by an A/D converter 16 and is inputted to a means 18 for deciding the measuring position. An instruction for taking in the data on the rate of displacement is inputted to a means 19 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 21 for calculating the rate of distortion for every specified angle, where the reference plane is set and the rate of distortion with respect thereto is calculated. Said rate is decided and if the rate is within a specified rate, the rate of correction is calculated and is graphically plotted by an X-Y plotter 24 via a means 23 for plotting the rate of distortion and the rate of correction.