To reduce facility cost or operational cost and to prevent occurrence of a crack by supplying atomized water or oil to a medium route of a cylinder as cooling medium of the cylinder to cool the cylinder.

The cylinder 1 has a structure of penetrating an extrusion route 3 for containing and melting material chip in a lengthy block 2, supplies the air or the air and atomized water to a medium route 5 overheated by a heater 4 arranged on an outer surface of the block 2 and embedded in a zigzag manner in the block 2 to cool it. That is, the air is always fed under pressure from a pressurized air supply source 7 to a supply route 6 coupled to the route 5. A water channel 8 is coupled to the route 6 via a coupler A with an orifice as a barrier. When a temperature sensor 10 attached to the cylinder 1 senses a predetermined temperature or higher, a valve 11 is opened, water is started to be supplied from a wafer supply source 12 and atomized water generated by atomizing is mixed with the air fed under pressure. Thus, an operation of a cooling system can be simplified and maintenance cost can be reduced.