Fundamental study was made to construct a hybrid power system composed of passive-type polymer electrolyte fuel cells (PEFCs) and electric double-layer capacitors for agricultural electric vehicles. For numerically simulating the behavior of the hybrid system, a fuel cell equivalent circuit model was developed and the model parameters were determined based on an electrochemical theory and experimental results. By means of numerical simulation with this fuel cell equivalent circuit model which can represent the actual current-voltage characteristic reasonably well for a passive-type PEFC unit, the efficiency of the passive-type PEFC controlled by the authors' original power control method named Time-Splitting Method (TSM) has been investigated under various conditions (TSM frequencies, duty ratios and external load resistances), and the improvement of the efficiency of this PEFC with TSM has been confirmed.