To enable demolding to be done without decreasing the productivity even in the tease where a relatively large undercut is formed in its inner surface.

This mold is provided with first and second slidable cores 3, 4 for forming the hollow part of a molded item provided between a mutually separable first mold 1 and second mold 2, wherein an elastic body 6 such as a rubber lying within the cavity encircled by the first and second molds 1, 2 and first and second cores 3, 4 is provided for forming an undercut in the inner surface of a molded item 7 on the outer periphery of the first and/or second cores 3, 4.