PURPOSE: To deduce automatically changeover timing of injection speeds and injection pressures to be required directly before completion of filling of resin, by a method wherein a quantity of change to a time of a quantity of mold break in molding cycles each is calculated and an injection speed or injection pressure are corrected by comparing the quantity of change with a predetermined set up value.

CONSTITUTION: When a quantity of mold break is taken as L in the section up to a changeover to a dwell process from a start of injection, a control device 20 performs sampling of the quantity of mold break detected by a position detector 9 at an arbitrary sampling cycle which is sufficiently shorter than a cycle time at every molding cycle. Then when a difference between a quantity of mold break L n at a certain time and a quantity of mold break ΔL (n+1) of the next cycle is taken as ΔL, a ratio between the L and a difference ΔL(n-1) which is one difference before the, ΔL is calculated. When the ratio enters a range set up by a setting part 25, such a control operation that an injection speed is changed over to a low speed or injection pressure is changed over by applying the brake to an injection system.