PURPOSE: To obtain a real operation characteristic curve with excellent precision and contrive to shorten a measuring time by a method wherein each measuring point of a real operation characteristic curve is obtained in an inspection straight line intersected at a specific angle and in a theoretical operation characteristic curve.

CONSTITUTION: Two intersections in a theoretical operation characteristic curve are specified by an input unit, and an inspection straight line 3 is calculated by algorism equipped in the input unit. This straight line 3 intersects at an angle α with a curve 1 at an intersection point 2, but in order to precisely acquire an intersection 2, the angle α is selected in the range of about 60 to 120°. In order to trace a measuring point 4 in a real operation characteristic curve 5 in each of lines 3, starting from the intersection 2 between the straight line 3 and the curve 1, a pair of points 6 to 8 in the straight line 3 having deviation which can be preliminarily selected are sequentially measured on both sides of the straight line 3, respectively, and when measurement results are different in the pair of points respectively, and measuring limit is within the last pair of points, the accurate measuring limit is repeatedly searched between the points. The obtained measuring limit corresponds to a measuring point 4 in a curve 5.