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Newly developed gas-diffusion electrodes (GDE) with binary catalyst composed of Ag and Pd were evaluated in a zero-gap type chlor-alkali cell. It was found that the cell voltage was 2.06V at 6kAm-2, which was 120 mV smaller than those of composed of single Ag catalyst or Pd catalyst. The increase of the cell voltage with the binary catalyst after 600 days of operation was only around 20 mV, which is one third of that of Ag catalyst. The particles sizes of the former catalyst were not changed after the electrolysis, but the size of Ag in the latter became large, which quantitatively corresponded to the increase of overvoltage predicted by a porous GDE model calculation. It was suggested that the agglomeration of the Ag catalyst could be inhibited by the existence of Pd.