A method of manufacturing of the present invention is a method of manufacturing a toothed belt (10) having a belt part (13) and a tooth part (14) sticking out of the belt part (13). This method of manufacturing includes a first step of forming a reinforcing-sheet (12) including a fiber sheet and a coating film formed on the fiber sheet by drying the fiber sheet on which an aqueous treatment agent containing a first rubber latex and a first vulcanizing agent is applied, and a second step of reinforcing a surface of the tooth part (14) with the reinforcing-sheet (12) by vulcanizing the first rubber and a second rubber at the same time while adhering a rubber composition, which is a material for the tooth part (14), including the second rubber and a second vulcanizing agent to the reinforcing-sheet (12). The aqueous treatment agent contains the vulcanizing agent in the range of 10 parts by weight to 100 parts by weight for 100 parts by weight of the first rubber.