PURPOSE: To provide a method for predicting the accurate driving current through single-time calculation without repeating the troublesome calculation.

CONSTITUTION: Step a: The relation of the DC current amplification actor to collector current is found. Step b: The relation of the ratio between the DC current amplification factor to collector current and the DC current amplification factor to constant current is graphed based on the relation of the DC current amplification factor to collector current. Step c: The driving current at the time of a driving voltage that a transistor(TR) uses is found assuming that the DC current amplification factor is a constant one. Step d: Values of the driving current found at the step c are plotted on the graph obtained at the step b and connected to the point 0 (origin) of the collector current by using straight lines. Then, step e: the driving current is found by reading the collector current at intersections of the relation graph obtained at the step b and the straight lines of the step d.