PURPOSE: To improve the accuracy of product dimension by stopping advance of a mandrel before arriving at the advance limiting position of a stem and starting the regression of mandrel at the time when it arrives at the advance limiting position of the stem.

CONSTITUTION: A stem position detecting sensor 8 and a mandrel position detecting sensor 9 are disposed at the upward of the mandrel 1 of a container 10 respectively. Further, piping systems of high and low pressure supplied to a hydraulic pressure switching part 13 are connected to a main cylinder 4 through solenoid switching valves 14, 15 and connected to a mandrel cylinder 6 through switching valves 16, 17. When the pipe extrusion becomes near the finishing point, the mandrel ram 5 is stopped at first by a striker 7 and sensor 9, then the retreating of a ram 5 is started through the switching valves 16, 17. At the time of cooling of the products, the effects of the mandrel ram 5 on the products cooling are prevented. By this method, the accuracy of the product dimensions is improved.