PURPOSE: To sense the completion of purging securely and automatically by sensing the quantity of residual resin from the scale of resin pressure generated by the degree of amount of residual resin in a cylinder and completing purging when the resin pressure becomes the given value or less.

CONSTITUTION: When the set given time has passed, a CPU 114 for PMC reads out the current resin pressure sensed from RAM 108 by a pressure sensor 5. Resin remains in a cylinder 2 and resin extruded by the rotation of a screw 1 are stored at the end of a nozzle, and melting pressure of said resin is applied to the screw 1 and the pressure applied from the resin is sensed by the pressure sensor 5. When the sensed resin pressure is determined to be the set value or lower, an instruction for stopping screw rotation is output to a CPU 112 for NC by putting the CPU 114 for PMC through BAC 113 and RAM 105 used in common to stop the rotation. Purging is completed by said arrangement.