To obtain a method for manufacturing a fuel tank which is made of synthetic resins and has excellent gas-barrier properties, and an apparatus for the method.

In the method for producing the fuel tank 1, an upper tank 3 and a lower tank 2 each having a substrate layer and a barrier layer are molded by injection-molding a synthetic resin for the substrate layer and a synthetic resin for the barrier layer respectively, and the molded upper and lower tanks 3 and 2 are united and fused together simultaneously by injecting a synthetic resin into its rim opening part to produce a hollow body. By this method, the fuel tank can be manufactured efficiently by the simple apparatus.