Temperature of power generating cells is measured, and, if the measured temperature is lower than a preset standard operating temperature, control is performed for zeroing an allocated amount of cooling air via an air allocation mechanism. During a startup period, cooling-air introduction openings are closed with reacting-air introduction openings kept open and an air-sucking-in fan activated. Thus, the entire amount of introduced air is caused to flow through the reacting-air introduction openings into the cells for reaction with hydrogen with a minimized cooling effect so that the power generation reaction can be promoted, which can reduce a time necessary for the cell temperature to increase up to a predetermined operating level. By constantly performing the control for zeroing the allocated amount of cooling air during the startup and subsequent periods, the power generating cell temperature is constantly allowed to rise readily as desired.