Phosphoric acid fuel cells have excess electrolyte in each cell to compensate for acid loss during operation. We studied experimentally the distribution of excess electrolyte between the anode ribbed plate and the anode substrate. It is found that the amount of excess electrolyte is balanced by the capillary force between the ribbed plate and the substrate. Next, we estimated the cell voltage with various amounts of excess electrolyte. The cell voltage is not influenced by the excess electrolyte when the acid fill level of the anode substrate is within 30%. But hen the acid fill level of the anode substrate is over 40%, the cell voltage is influenced by the excess electrolyte ill the anode substrate. In this case the diffusion coefficient of anode substrate is was estimated 1/10 of that of when the acid fill level is 0%. We concluded that the amount of excess electrolyte Must be set Such that the acid fill level of anode substrate is within 30%. To increase the amount of excess electrolyte in the cell. We must Chose all anode ribbed plate for which the capillary force is larger than that of the anode substrate.