The decarbonylation of a cage-opened C60 derivative was examined by employing single-electron reductants. During the reaction, an H2O molecule was spontaneously encapsulated inside the cage (up to 78%) through the thus-formed 14-membered-ring orifice even though the H2O encapsulation had long been considered to require an orifice consisting of at least 16 atoms. The crystallographic analysis revealed an orifice shape closer to a circle which significantly contributes to the decreased activation barrier for the H2O encapsulation.