The proton conducting properties of layered metal phosphates hydrates (M (HPO4)2·nH2O (M = Sn, Ti, Zr), MPs) were examined under various humidities at 20−150°C. SnP showed a high conductivity of more than 10-2 S cm-1 under saturated water vapor pressure and a weak humidity dependence of its conductivity at 150°C. The hydrated water content (n) of ZrP was approximately stable at 1 in a wide humidity range of more than 20% at 150°C. The n of SnP varied more widely than that of ZrP; however, the conductivity of SnP remained high even in a low-n range. The short distance between adjacent hydrated water molecules was suggested to be the reason for the high proton conductivity of SnP.