We have analyzed the impedance of the cathode of a polymer electrolyte fuel cell (PEFC) to estimate oxygen diffusion distance, δ. Assuming planar oxygen diffusion in a thin liquid water layer at the cathode unconventionally, we can regard δ as water layer thickness adjacent to the cathode catalyst layer. Calculated values of δ agree well with our previous experimental result of liquid water accumulation amount at the cathode. Measurement of impedance variation with time reveals that δ initially tends to be almost constant with time after start-up from dry condition, but that excessive water accumulation due to long time operation causes an increase in δ. From the viewpoint of the diagnosis, an exceeding increase in δ can be used to indicate the flooding at the catalyst layer.