To eliminate strength poverty of a weld line part, jetting and floating of glass fibers when injection-molding resin.

In a cavity C formed between a stationary mold 16 and a movable mold 32, the resin 34 is injected from two gates 22A, 22B. When injecting the resin 34, by forward and backward movement of a slide core 38 in the inside of a molding hole 36 forming the cavity C, the cavity C is kept at a constant volume until after elapse of a fixed time from start of the injection. After the fixed time elapses and the resin 34 in the cavity C merges and closely contacts with each other, the slide core 38 is gradually moved backward until completion of the injection to gradually expand the volume of the cavity C. The forward and backward movement of the slide core 38 is performed by a slide core forward and backward movement mechanism 80. With this, a weld line in a product 120 is prevented from being generated and strength poverty is eliminated. Further, jetting and floating of glass fibers when using material containing glass fibers are eliminated.