To minimize the drop of temperature by making a power feeding cable to heat and warm a die cassette freely moving as it is connected to the die cassette between the position at the outside of a machine and the using position of the inside of the machine.

A die cassette 24 mounting a next die 26 is held on a machine outside waiting position, a heater 29 is energized and the next die cassette 24 is pre-heated. The machine inside using position is positioned with a die mounted on a die cassette 24A. By extending a piston rod of a due cassette advancing and retracting cylinder, the die cassette 24 is advanced to the machine outside waiting position as mounting the die 26. Next, as the die 26 is mounted, the die cassette 24 is moved to the machine outside waiting position, and it is arranged in tandem on a die changing device. Next, the die changing device is moved to the direction of extruding the product, the die cassette 24A of mounting the die is occupied at the machine outside waiting position and the die cassette 24 of mounting the die 26 is waited outside the machine at the same time.