PURPOSE: To obtain a rubber reinforcement which is effective for the rationalization of production processes by a method in which a compound having 20% or more swelling rate for unvulcanized rubber and an adhesive composed of rubber, etc., are adhered to the surface of a thermoplastic synthetic fiber.

CONSTITUTION: One hundred pts.wt. mixture of adhesive composed of a rubber, a resorcinol-aldehyde precondensate, and aldehyde compound and a compound having 20% or more swelling rate for unvulcanized rubber is mixed with 0.5W 15pts.wt. an emulsifier, and the mixture is dissolved or dispersed in water to prepare an aqueous composition. The aqueous composition is adhered to the surfaces of a thermoplastic synthetic fiber composed of an aliphatic polyamide, polyester or an aromatic polyamide to obtain a rubber reinforcement. As said rubber-swelling compound, butyl stearate, octyl stearate, etc., may be cited.