A fuel cell is operated with high power such that which a humidified gas and a dry gas are selectively supplied as oxidant to a cathode of the fuel cell. This method includes (S1) supplying a humidified gas while a power is constantly maintained or until the power begins to decrease; (S2) after supplying the humidified gas, supplying a dry gas to obtain a greater power than an average power of the step (S1); and (S3) after obtaining a predetermined power in the step (S2), repeatedly supplying a humidified gas in case the power decreases and supplying a dry gas in case the power decreases again afterwards, thereby increasing the power such that the predetermined power is maintained. This method provides an optimal operating condition to a fuel cell, thereby ensuring a high power.