A space 12 is formed at a tip portion of a nozzle 1 so as to decrease a thermal transfer of the nozzle 1. Alternatively, a center portion of one of a semi-spherically protruded surface of the tip portion of the nozzle 1 and a semi-spherically recessed surface of a sprue bush protrudes so that the radius of curvature of one semi-spherical surface is the same as the radius of curvature of the other semi-spherical surface. As a result, since the nozzle 1 and the sprue bush 2 are surface-contacted, heat of the tip portion of the nozzle 1 absorbed to the sprue bush 2 is increased. Thus, the temperature of the tip portion of the nozzle 1 can be lowered easily. After the resin material injected into the molding cavity 11 is cooled, when the sprue portion is removed from the fixed die, a tip portion of a rod-shaped protrusion portion of the sprue can be properly cut.