An impermeable rubber hose is thinned for the purpose of reducing weight and costs. A rigid resin film is used as an intermediate layer. When the resin film is gradually rolled up into a cylindrical shape from a planar shape and fed to an extrusion head and rubber is simultaneously extrusion-molded inside and outside the cylindrical film, the film produces a break or a twist to make extrusion molding difficult. It is an object of the present invention to make simultaneous extrusion molding possible even in such a condition. To attain the above object, a thermoplastic resin film (5) which forms an intermediate layer (2) and of which the melting point is lower than the vulcanizing temperature is formed in a long belt-shape. Slits (7) are provided at fixed intervals on the right and left long side sections (8, 8) of the film (5) in the longitudinal direction. When the resin film (5) is fed into an extrusion head (10) to be rolled up into a cylindrical shape, because both the long side sections (8, 8) of the film (5) are formed as weak sections by the slits (7), the film (5) adsorbs a strain when bent and is bent in a circle, thereby allowing the right and left long side sections (8, 8) to overlap at an overlapping section (6). In this condition, rubber is simultaneously extrusion-molded inside and outside the film and then, vulcanized to weld the overlapping section (6), thereby eliminating the slits (7).