Resin-made container manufacturing method, manufacturing device, and metal mold unit

A manufacturing method of a resin container, the manufacturing method comprising:

• a first injection molding process for injection-molding a first layer of a preform by using a first resin material, the first layer of the preform having a bottomed cylindrical shape and including a groove that extends in an axis direction;
• a second injection molding process for injecting a second resin material into the groove of the first layer, the second resin material having a color that is different from a color of the first resin material, and laminating a second layer on an inner peripheral side of the first layer;
• and
• a blow-molding process for blow-molding a preform that includes multiple layers and has been obtained in the second injection molding process in a state where residual heat at a time of injection molding is contained, and manufacturing the resin container having a vertical-striped color pattern formed by using the second layer,
• wherein, in the first injection molding process, a first core mold having a groove formed therein is used to form the groove on the inner peripheral side of the first layer, and
• in the second injection molding process, a cavity mold that holds a shape of the first layer is disposed on the outer peripheral side of the first layer, and the second resin material is injected between a second core mold that is different from the first core mold and the groove of the first layer.