for 7 hours and thus platinum was supported on the carbon powder then the solution was filtered and the thus treated carbon powder was dried to yield a platinum catalyst having a supporting density of 50 the annealing treatment was performed by maintaining the platinum catalyst produced by the foregoing step in 100 hydrogen gas for 1 hour at 900 u00b0 c the platinum catalyst subjected to the foregoing heat treatment was subjected to an oxidizing solution treatment for the addition of the hydrophilic group the heat treated catalyst was treated for 2 hours in a 3 0 mol l nitric acid aqueous solution the amount of the dissolved oxygen 0 003 cm 3 cm 3 in stp at 80 u00b0 c and then filtered and dried for the foregoing steps of producing the catalyst the platinum particle size the specific surface area of platinum and the amount of the bonded hydrophilic group were examined after each of the steps of the supporting of platinum the heat treatment and the acid treatment the measurement of the platinum particle size was performed by x ray diffraction analysis the specific surface area of platinum was determined on the basis of comsa and the quantitative determination of the hydrophilic group used a.