The proton conductive property of methano [60] fullerene diphosphoric acid has been investigated under various humidity conditions at the temperature range between 15 and 45 degreesC. It shows proton conductivity as high as 10(-2) S cm(-1) at 25 degreesC under relative humidity of 95%. Thermal analyses including TG-DTA and thermal desorption mass spectroscopy (TDS) confirm that the compound is thermally stable up to 200 degreesC. Proton conduction of the compound depends very much on humidity or water content. The logarithmic conductivity at 25 degreesC is increased linearly with increasing relative humidity. The activation energy (E-a) estimated from the slope of log(sigmaT) vs. 1/T is decreased from 1.08 to 0.52 eV, as the relative humidity is increased from 40% to 75%. The humidity dependence of conductivity is discussed in the light of the observed hydration isotherm.