The dependence of the ion-temperature-gradient scale length on the hydrogen isotope mass was examined in conventional H-mode plasmas in JT-60U tokamak. While identical profiles for density and temperature were obtained for hydrogen and deuterium plasmas, the ion conductive heat flux necessary for hydrogen to sustain the same ion temperature profile was two times that required for deuterium, resulting in a clearly higher ion heat diffusivity for hydrogen at the same ion-temperature-gradient scale length. On the other hand, the ion-temperature-gradient scale length for deuterium is less than that for hydrogen at a given ion heat diffusivity.