Proton solid electrolytes, which exhibit high proton conductivity at a wide range of low-intermediate temperatures (150-300 °C), are key materials for the development of fuel cells for automobiles and cogeneration systems. In this study, a benitoite-type polyphosphate, KMg1-xH2x(PO3)·yH2O, which has a non-combustible and layered structure, was investigated as a new proton conductor. The benitoite-type KMg1-xH2x(PO3)·yH2O was synthesised by a coprecipitation method. The solid solution formed in the range of x = 0-0.100 in KMg1-xH2x(PO3)3·yH2O. Multi-step weight loss due to dehydration was observed for TG/DTA measurement at 30 °C and 150 °C. We observed enhanced peaks of the vibration bands at around 1117 cm-1 and 1229 cm-1, which were attributed to the symmetric and asymmetric PO2 vibration modes, and at 743 cm-1 and 970 cm-1 due to the ns(P-O-P) and nas(P-O-P) modes as well as broad absorbance peaks at 2300 cm-1 and 2700 cm-1 corresponding to the vibration modes of ns(P-O-H) with increasing x for FTIR spectra, which suggest the introduction of protons to the crystal structure. Proton conductivity increased from x = 0 to 0.10 and then decreased at x = 0.125, where the impurity phase was observed. The sample with x = 0.10 in benitoite-type KMg1-xH2x(PO3)3·yH2O exhibited high proton conductivity of 1.4 × 10-3 S cm-1 at 150 °C and 6.5 × 10-3 S cm-1 at 250 °C under a non-humidified N2 gas flow.