To restrain generation of sink marks without increasing a thickness of a base plate for the reason that a sink mark generates on a rear face of a boss resulting in damage of its appearance when the thickness of the boss exceeds 2/3 of the thickness of the base plate for improving the screwing strength of the boss.

A mold 1a for forming a face A, a mold 1b for forming a face B and a boss, and a pin 5 for forming a center hole of the boss are provided. There are also provided a boss-like movable mold 6 which fits in the outer peripheral face of the pin 5 for forming the center hole of the boss and an inner peripheral face of a cavity 3 of the boss, and a stopper 8 which temporarily interrupts the backing of the movable mold 6 due to injection pressure. Until a skin layer having a predetermined thickness is formed at a position 3a corresponding to a boss on the face A, the boss-like movable mold 6 is held by the holding of the stopper 8. After the skin layer is formed, the stopper 8 is released so as to make the boss-like movable mold 6 back by injection pressure.