A primary injection stroke of a trial shot, which precedes a continuous molding operation, is performed with a pressure regulating value maintained constant by speed feedback control from initiation of the primary injection stroke until a measured value of pressure reaches the pressure regulating value and, after the measured value of pressure has reached the pressure regulating value, the speed feedback control is switched to pressure-priority control under an open loop control and the primary injection stroke is performed up to a dwell pressure change-over point. An entire range of a primary injection stroke of the continuous molding operation is then performed by speed feedback control while using, as target speed data in the primary injection stroke of the continuous molding operation, a pattern of speed values measured during the primary injection stroke in the trial shot upon confirmation of molding of a non-defective product.