Disclosed is a tie coat which is formed on a surface of a base or an undercoating film prior to formation of an organopolysiloxane-based antifouling coating film and is formed from a moisture-curing organopolysiloxane-based composition comprising (b1) organopolysiloxane having condensing functional groups at both ends of a molecule and/or (b2) a curing composition formed by subjecting the component (b1) and an extender pigment selected from the group consisting of silica, calcium carbonate, talc, mica, clay, kaolin and barium sulfate to contact treatment with heating or without heating. Also disclosed is a composite coating film wherein on a surface of the above tie coat is formed a finish coat formed from a three-pack type organopolysiloxane-based curing composition comprising (c1) a main agent comprising the component (b1), (c2) a curing agent component comprising tetraalkoxysilicate or its condensate and (c3) a curing accelerator component comprising a metallic compound. Further disclosed is a composite coating film wherein an epoxy-based sealer coat, the tie coat and the finish coat are formed in this order on a surface of an old antifouling coating film (G). There is provided by the invention a tie coat capable of forming a composite coating film having excellent interlaminar bond property. The composite coating film is favorably formed on a surface of a base, an undercoating film, an old antifouling coating film or the like, has excellent interlaminar bond strength and antifouling property and is preferably used for coating outer surfaces of ships, submerged parts of marine structures, water supply/drainage channels of atomic power plant, etc.