A device for injection molding a preform in a shape of a test tube used in a biaxial stretching and blow molding operation, in which at least one intermediate layer is laminated with substrate layers made of a main resin, the device having a nozzle section for laminating the main resin with a resin that forms the intermediate layer, said nozzle section comprising: three cylindrical layer-forming flow channels disposed in a coaxial cylindrical configuration having tapered cylindrical portions at respective downstream ends, which include: an inner flow channel and an outer flow channel for forming the substrate layers of the main resin, and a middle flow channel located between the inner flow channel and the outer flow channel and used to form the intermediate layer made of an intermediate-layer resin, which resin is distributed in a molten state to the middle flow channel by way of a manifold that comprises groove-like conduits, and characterised by a predetermined number of vertical blocking rib pieces disposed only at the tapered cylindrical portion at an end of the middle flow channel on a downstream side, in parallel in a circumferential direction, so that the intermediate layer is segmented in the circumferential direction into a predetermined number of vertically running streams, wherein the flow behavior of the intermediate-layer resin flowing through the middle flow channel is adjusted by positioning the vertical blocking rib pieces in the circumferential direction, wherein the vertical blocking rib pieces are provided in a predefined number and shape depending on a maximum height point (Pmax) of a leading edge of the intermediate layer and are disposed in the circumferential direction in such a way that one of the predetermined number of the vertical blocking rib pieces is located at a circumferential position opposed by 180 degrees to a position of an inflow port, which is disposed at an upstream end of the manifold to allow the intermediate-layer resin to flow into the middle flow channel; wherein one of the vertical blocking rib pieces is located at a circumferential position corresponding to a position where the maximum height point (Pmax) of the leading edge of the intermediate layer of the preform is likely to appear. three cylindrical layer-forming flow channels disposed in a coaxial cylindrical configuration having tapered cylindrical portions at respective downstream ends, which include: an inner flow channel and an outer flow channel for forming the substrate layers of the main resin, and a middle flow channel located between the inner flow channel and the outer flow channel and used to form the intermediate layer made of an intermediate-layer resin, which resin is distributed in a molten state to the middle flow channel by way of a manifold that comprises groove-like conduits, and characterised by an inner flow channel and an outer flow channel for forming the substrate layers of the main resin, and a middle flow channel located between the inner flow channel and the outer flow channel and used to form the intermediate layer made of an intermediate-layer resin, which resin is distributed in a molten state to the middle flow channel by way of a manifold that comprises groove-like conduits, and characterised by a predetermined number of vertical blocking rib pieces disposed only at the tapered cylindrical portion at an end of the middle flow channel on a downstream side, in parallel in a circumferential direction, so that the intermediate layer is segmented in the circumferential direction into a predetermined number of vertically running streams, wherein the flow behavior of the intermediate-layer resin flowing through the middle flow channel is adjusted by positioning the vertical blocking rib pieces in the circumferential direction, wherein the vertical blocking rib pieces are provided in a predefined number and shape depending on a maximum height point (Pmax) of a leading edge of the intermediate layer and are disposed in the circumferential direction in such a way that one of the predetermined number of the vertical blocking rib pieces is located at a circumferential position opposed by 180 degrees to a position of an inflow port, which is disposed at an upstream end of the manifold to allow the intermediate-layer resin to flow into the middle flow channel; wherein one of the vertical blocking rib pieces is located at a circumferential position corresponding to a position where the maximum height point (Pmax) of the leading edge of the intermediate layer of the preform is likely to appear.