A method of operating a polymer electrolytic fuel cell, wherein its operating condition is regulated for an optimal one which is represented by a function of a gas flow rate and the difference between a saturated steam pressure and an actual steam pressure, by regulating an in-plane temperature distribution obtained by a cooling water flow direction and by the regulations of a cooling water inlet temperature and a cooling water flow rate, a gas supply amount, and a supplied moisture amount, and a current density, the method resolving conventional problems with a polymer electrolytic fuel cell that a performance distribution occurs in a single plane of a cell and a stable operation is difficult when a performance distribution is significant.