To provide a structure of a rubber support molding die which makes it possible to use a die widely corresponding to changes of the dimension and rubber quality of a rubber support and ensures the flatness of the side surface of the rubber support product, capable of preventing a defective dimension and flatness defect.

A rubber-outflow slit 13a to 13d, 14a to 14d having a required width, e.g. 0.5 to 1 mm, are formed at the ridge line part X1 to X4 of a spacer 8 with a lower die 4 and the ridge line part Y1 to Y4 at which the spacer 8 contacts with an upper molding plate 10. And, also, rubber reservoirs 15a to 15d, 16a to 16d, located upper and lower, with a required volume, interconnecting with the slit 13a to 13d, 14a to 14d, are formed at the upper molding plate 10 and the spacer 8.