The solid electrolyte of the present invention is composed of an organic/inorganic composite material having pores with a mean pore diameter of 1 to 30 nm and having a skeleton comprising a metal atom, an oxygen atom bonded to the metal atom, and an organic group having at least one carbon atom bonded to the metal atom or the oxygen atom, and a functional group having an ion exchange function and bonded to the organic group inside the pores. As a result, even if the relative pressure of the water vapor in the atmosphere is less than 1.0, it is still possible to achieve a solid electrolyte with a sufficiently high ion conductivity at a lower temperature than with a conventional solid electrolyte such as stabilized zirconia.