To provide a proton conductor functioning as an electrolyte membrane at high temperature of 100 to 200°C, and causing no problem such as oozing-out and volatilizing of an ionic liquid during usage; and to provide a polymer electrolyte fuel cell using the proton conductor as an electrolyte.

In order to fix a salt (that is, an ionic liquid) composed of a Bronsted base and a Bronsted acid, having a proton and having at least one heteroatom, a polymer is useful which has a substituent having active hydrogen such as a hydroxyl group. The proton conductor is composed of a compatibilized product of (1) the ionic liquid and (2) the polymer having a substituent having active hydrogen such as a hydroxyl group. The example of such polymer includes: polymers formed by polymerization of at least one monomer selected from the group consisting of maleimide compounds and vinyl compounds; and celluloses such as bacteria celluloses.