To provide an injection molding method, in which a sufficient amount of a resin is filled in a thin molded part and no gas-burnt part nor flowing pattern is generated in a thick molded article even when a thin molded article and a thick molded article are produced by one die (e.g. a multi-layer molded article is produced by multi-layer injection on a thin molded article) at one and the same shot time with the use of a conventional injector having one injection cylinder.

A cavity for a thin molded article and a cavity for a thick molded article are equipped in one injection molding die; an open gate type hot runner is used in the cavity for a thin molded article and a bulb gate type hot runner is used in the cavity for a thick molded article; and a thin molded article and a thick molded article are produced at the same time by opening and closing the bulb during injection.