To provide a manufacturing method of a cast elastomer belt which has both a cog portion and a V rib portion on a belt inner surface, and in which demolding can be easily performed.

The manufacturing method of a cast elastomer belt 22 including a cog portion area 18 having a cog portion and a grooved portion and a rib portion area 17 having a plurality of ribs 14 extending in the longitudinal direction of a belt on the inner surface comprises: inserting an inner mold 5 wound around with a core wire 10 in a spiral shape into an outside mold 4 wherein a plurality of grooves 20 are engraved along a circumference direction and a plurality of grooves 21 extending in a longitudinal direction so that the grooves 20 of the circumference direction may be separated, a step of filling up a space portion 6 formed between the outside mold 4 and the inner mold 5 with a liquid elastomer, followed by curing; and after that, taking out one mold and releasing a molded belt body 8 adhering to the outside mold 4.