Abstract: In the present invention, a preliminary extruding operation is carried out for each section of a movable die moving pattern DA corresponding to the configuration of an article, thereby determining a suitable back pressure at which a desired cross-sectional dimension is obtained. The movable die is secured to a position DA1 corresponding to a side section of the article, then screw revolutions are changed while extruding a molding material. Thus, determined as a suitable back pressure BPO for the side section is a specified value of the back pressures BP at which the molded product has the desired cross-sectional dimension. Likewise, a suitable back pressure for the upper section is also determined, so that a pattern of the suitable back pressures BPO is determined. In view of the time delay, timings of change in an initial setting pattern R1 for the screw revolutions are advanced for periods of times T.sub.11, T.sub.12, T.sub.13 and T.sub.14 from the timings of change in the pattern DA.