We are developing a method of diagnosing fuel-cell deterioration in phosphoric-acid-fuel-cell (PAFC) power plants that use a small single cell, based on measuring its current-voltage (I-V) characteristics. As the fuel-cell deteriorates, the coefficient k in the linear equation (V=kI+V0), which approximates the I-V characteristics, increases and the constant V0 decreases. Thus k and V0 can be used for diagnosis as well as the cell voltage. The ratio of the maximum power to the rated power PMAX/PR approaches 1.0 as fuel-cell deterioration proceeds. Therefore, PMAX/PR can be used as a parameter for predicting fuel-cell lifetime when the minimum output power PMIN during fuel-cell power generation is given.