A proton conducting film includes a polymer having a first part and a second part which are connected by a covalent bond and a plasticizer. The first parts aggregate with each other to form a domain at an operation temperature of the proton conducting film, and the second part crosslinks the domains. The second part has a proton accepting group, and the plasticizer contains a proton donating compound having a pKa of 2.5 or less, and thus the plasticizer penetrates into the second part, and a glass transition temperature of the polymer is lowered compared to when the plasticizer is not included.