PURPOSE: To measure the deflection to a straight line connecting two points, by setting a measuring device that has a measuring element and can measure the coordinates by a rectangular coordinate system within a cut plane passing through two points on a free curved surface and giving a coordinate conversion to the obtained coordinate value.

CONSTITUTION: A slider 4 is moved so that a measuring element 5 is positioned at a free measurement starting point Ps on a free curved surface of an object W to be measured on a stand 9. Then a button 24s is pushed to supply a signal to a CPU11, and counters 7 and 8 are set at 0. Then the original point is set at the start point Ps for a rectangular coordinate system of a measuring device which uses coaxial lines Xm and Zm as coordinate axes. Then the element 5 is moved to a terminal point Pe, and a button 24e is pushed. Thus the corresponding coordinate value is set at the counters 7 and 8 respectively. The angle θ formed by a straight line Xr connecting points Ps and Pe and the axis Xm is calculated, and a coordinate conversion is performed to a rectangular coordinate system (Xr, Zr). With a push of a button 24m, the slider 4 is moved at a fixed speed from the point Ps to the point Pe. Then the deflection is measured to the Xr.