A piston head of a hydraulic injection molding machine is screwed with a
tie rod inside a hydraulic cylinder of the hydraulic injection molding
machine. The piston head has a second screw formed in an inner
circumference of a hollow of the piston head and engaged with a first
screw formed in an outer circumference of the tie rod. An insert groove
having a predetermined shape is formed in the inner circumference of the
piston head in front of the second screw.