To provide a method for producing a transmission belt which can suppress the damage of a core wire and exactly etch ribs.

After a stretched rubber layer onto which an unvulcanized stretched rubber sheet is wound is arranged in an inner type cylinder drum fitted with a flexible jacket, a twist cord with an upper twist coefficient of 3.0-10.0 is spun spirally, the compressed rubber layer is arranged by winding the unvulcanized compressed rubber sheet, and an unvulcanized belt sleeve is formed. Next, after the cylinder drum is mounted on the inside of an outer type mold and fixed, by expanding the flexible jacket, the unvulcanized belt sleeve fitted to the peripheral surface of the jacket is expanded uniformly in the radial direction to press the unvulcanized belt sleeve to the V-shaped projection of the outer type mold so that a vulcanized belt sleeve having a plurality of V-shaped grooves on the surface is formed.