PURPOSE: To enable a plastic molded item to be divided easily without damaging the configuration, by setting a weld plane and the central plane of a groove for dividing approximately on the same plane so that the divided cross section may be made smooth.

CONSTITUTION: The configuration of an interposed section 8a and the area through which an injected material is passed are selected suitably, so that the fluid state from a central path 8 and the fluid state from opposite side sections 6 are controlled, and if required the configuration, etc. of an interposed section 7a are controlled. The injected material flows into the interposed section 7a from the opposite sides under approximately same conditions depending on the distance over which the injected material has flowed to the section 7a and the loop resistance, so that a weld plane 10 is formed approximately on the same plane as the central plane 11 of a groove 7. When the injection molded plastic item is divided along the groove 7, it will match with the weld plane 10.