An injection molding device including a middle flow channel which forms an intermediate layer of a preform which is subject to biaxial stretch blow molding. The middle flow channel s segmented into a predetermined number of flow sub-channels by vertical blocking rib piece(s). Side wall's upstream portions of the vertical blocking rib piece are formed as angled surfaces whose distance from each other decreases from the downstream side to the upstream side, and/or, an end portion of the vertical blocking rib piece that is located on the downstream side is tucked in at least one of the inner flow channel and the outer flow channel.