NOVELTY - A proton conductor comprises an ionic liquid retained in a matrix composed of an acidic group-containing solid polymer, at a ratio of (30-90) wt.% with respect to the weight of matrix. The ionic liquid includes a cation and an anion, which are subjected to ionic bonding; and is liquid at room temperature. The retention ratio of the ionic liquid in the matrix, obtained after passage of 24 hours when immersed in water is not less than 50%.    USE - As an electrolyte for various electrochemical cells such as fuel cells (e.g. hydrogen fuel cell and direct methanol type fuel cells) and electrolysis apparatus (for electrolyzing water to produce hydrogen and oxygen).    ADVANTAGE - The matrix of the proton conductor has an excellent ability to retain a large amount of ionic liquid of (30-90) wt.% of the acidic group-containing polymer without disengagement, as compared to the prior art proton conductors. Further, the retention ratio of the ionic liquid in the matrix, obtained after passage of 24 hours when immersed in water is (not less than 50, preferably not less than 90, especially 100)%. Thus, the amount of the ionic liquid retained is not affected even in the presence of water that is produced at the cathode after the operation of the fuel cell, without outflowing of the ionic liquid and the conductor maintains the proton conductivity. Due to the excellent liquid retention ability, the proton conductor does not require any additional equipment for maintaining its performance by prevention of drying, such as a humidifier or a cooling system, when used in electrochemical cells, and allows the construct a compact fuel cell having a simple structure. This improves the performance of the proton conductor, and ensures excellent power generation performance for an electrochemical cell over a long period of time, even at an operation temperature of not less than 100 degreesC and/or when the cell is operated in a low humidity environment. Thus the proton conductivity obtained when the fuel cell is operated in a high temperature low humidity state, is substantially equivalent to the proton conductivity obtained when the fuel cell is operated in a low temperature high humidity state. The method for the construction of the proton conductor is easy and convenient and avoids cracks, scratches or aggregation produced in the material.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for production of a proton conductor by:    (a) dissolving an acidic group-containing solid polymer having an acidic group in a solvent, at a ratio of (1-20) wt.% of the solvent to prepare a solution;    (b) adding an ionic liquid that contains a cation and anion that are subjected to ionic bonding and remains liquid at room temperature; to the solution at a ratio of (30-90) wt.% of the polymer; and    (c) obtaining the proton conductor by performing casting with the solution.    The solvent is an organic solvent other than methanol.