PURPOSE: To ensure that no trailing drip occurs, an extrusion rod does not move backward excessively and no extra force for moving the extrusion rod is required.

CONSTITUTION: A power cutoff means for disengaging an extrusion rod 6 with a power source is provided in a power transmission mechanism for transmitting power from the power source to the extrusion rod 6. In addition, a resistance means 28 is provided which disengages the extrusion rod 6 with the power source, interlocked with the power cutoff means and applies a specified load to the extrusion rod temporarily. Further, the extrusion is disengaged with the extrusion rod by the power cutoff means and a resistance is applied to the extrusion rod. This resistance has only to be applied temporarily immediately after the disengagement.