PURPOSE: To contrive improvements in productivity and molding quality by making the optimum gain setting easy, by a method wherein a correction coefficient between a set gain coefficient and calculated gain coefficient is obtained, gain adjustment is performed on the basis of the set gain coefficient selected on the basis of an analogic detected signal of a physical quantity and gain correction is performed by the correction coefficient.

CONSTITUTION: At the time of gain setting, the optimum set gain coefficient Km in a gain coefficient group 5 is selected by a control part 6. Namely, the actual optimum calculated gain coefficient Kp is obtained by operation. Then a correction coefficient Ak between the set gain coefficient Km selected by the control part 6 and the calculated gain coefficient Kp is obtained by the operation and stored. Therefore, an analog detected signal Sa output through an inner pressure sensor 2 of a mold is amplified, gain-adjusted by the set gain coefficient Km, converted into a digital detected signal Sd and corrected by the control part 6 through the correction coefficient Ak. Thus the optimum gain setting can be performed easily and highly precisely.