PURPOSE: To carry out more precise adjustment by computing the optimum variable degree of a first characteristic value successively based on immediately prior two measurement points shown as results for every adjustment including the dispersion of electric equipment themselves.

CONSTITUTION: As initially measured values, OT (a first characteristic value), Pi (a working voltage), Do (a second characteristic value) are measured by prescribed measuring instruments to give initial measurement points. Then, based on the initial measurement points, OT is initailly adjusted as to make it a previously set standard value while assuming the Pi and Do would be aiming values and then OT, Pi, and Do are measured and the measurement points of the second time are judged whether they are within a prescribed range. The optiomum variable degree of OT is computed and corrected time to time based on the immediately prior two measurement points shown as the results of every adjustment always including the dispersion of electric appliances themselves which are subjected to adjustment and the variable degree of OT is corrected. Adjustment can be carried out more precisely and more quickly by carrying out adjustment while carrying out correction.