Friction and to stabilize the electric behavior, for a power transmission belt that further improve the run life, provided for the fiber-containing rubber composition, providing a short-fiber for reinforcing rubber. As the adhesive synthetic sheet using only dispersed in cutting is water's multifilament fiber, rubber fiber composition, of said single fiber, and the following, characterized in that the proportion of the adhesive raw count single yarn (單 絲) of 0 - 5% step: a step of cutting a cord woven of synthetic filament multifilament yarn cord fabric was dipped in an adhesive treatment liquid, a step of pressing a nip of 1.2~5.0 ㎫ by a nip roll, and the bundle of the predetermined length of the cord fabric obtained the process for producing a monofilament comprising a.