A method of manufacturing a rubber weir main body, which manufactures a rubber weir main body intended to rise by the flow-in of a fluid and to fall by the discharge of the fluid by integrally bonding at least two main body segments divided in the circumferential direction to each other, including the steps of: forming a material removed portion with stepped portions at edges of the rubber weir main body segments previously manufactured; abutting the rubber weir main body segments on each other at the edges; laminating unvulcanized rubber sheets containing reinforcing canvasses and an unvulcanized rubber sheet on the material removed portion formed between both the rubber weir main body segments contacted with each other; and integrally vulcanizing and bonding them. With this method, a large sized rubber weir main body excellent in quality can be easily manufactured at a job site.