Abstract: A screw head is composed of a cone portion, a tapered portion formed behind the cone portion and a narrow portion formed behind the tapered portion. A counter-flow stopper ring is provided around the narrow portion so that an annular resin passage is formed between the counter-flow stopper ring and the narrow portion. The tapered portion of the screw head has a plurality of circumferentially spaced grooves. The sectional area of each groove decreases from the inner circumference toward the outer circumference of the tapered portion. The grooves are so formed that the resin flowing through the grooves creates a force which pushes the counter-flow stopper ring backward, the ring being moved forward by forward flow of resin caused during backward movement of the screw. Thus, the grooves help maintain a gap between the counter-flow stopper ring and the screw head.