To form an adhesive layer having a good adhesion to an unvulcanized rubber and excellent in adhesion to a vulcanized rubber while keeping good working environment without using an organic solvent by incorporating a liquid elastomer or a tackifier.

As a solvent-free cement is employed one containing as a main component a liquid elastomer or a tackifier. The liquid elastomer is suitably selected among liquid natural rubbers, liquid polyisoprene rubbers, liquid polybutadiene rubbers and the like according to the use. The tackifier is suitably selected among those known hitherto as tackifier for rubbers such as rosin- based, terpene-based, petroleum resin-based tackifiers and the like according the use. This solvent-free cement is applied on the surface of an unvulcanized rubber material in an amount of around 0.1-0.3 g/100 cm2, to which an unvulcanized rubber material is closely stuck and subjected to heat treatment under a pressure to effect covulcanization, whereby the vulcanized rubber material can be strongly adhered through the vulcanized cement layer.