NOVELTY - A hydrogen sensor (1) has a substrate (2), a rare-earth-metal film (3) formed on the substrate and a protective film (5) formed on the rare-earth-metal film. The protective film has a protective film formed by dispersing hydrogen permeability metal particles into a ceramic material and a pair of electrode film formed by mutually opposing with a predetermined space on the protective film. A water-repellent film (4) is formed on the protective film.    USE - Hydrogen sensor is used for detecting low concentration hydrogen gas leakage from apparatus such as fuel cell for motor vehicles.    ADVANTAGE - The hydrogen sensor having excellent durability and water resistance is manufactured economically. The hydrogen sensor is stably operated in high concentration atmosphere.    DETAILED DESCRIPTION - A hydrogen sensor has a substrate, a rare-earth-metal film formed on the substrate and a protective film formed on the rare-earth-metal film. The protective film has a protective film formed by dispersing hydrogen permeability metal particles into a ceramic material and a pair of electrode film formed by mutually opposing with a predetermined space on the protective film. A water-repellent film is formed on the protective film. The water-repellent film is formed with a fluorine resin material having water-repellent property and a silicone type material. The rare-earth-metal film contains yttrium, cerium and/or lanthanum. The hydrogen permeability metal particles contain palladium, platinum, niobium, vanadium and/or tantalum. The ceramic material contains nitride or oxide of group (IVa, Va and Via) elements such as aluminum nitride (AlNx), aluminum oxide (AlOy), silicon nitride (SiNz) and/or silicon oxide (SiOr), where x is 0.5-1, y is 0.8-1.5, z is 0.7-1.3 and r ix 1-2. The substrate is a glass plate or a ceramic board. The hydrogen sensor is provided with a heater which is a resistance film containing platinum, ruthenium oxide or silver-palladium alloy film on a substrate. An INDEPENDENT CLAIM is included for manufacture of hydrogen sensor.    DESCRIPTION OF DRAWING(S) - The drawing shows the sectional view of the hydrogen sensor.    Hydrogen sensor (1)    Substrate (2)    Rare-earth-metal film (3)    Water-repellent film (4)    Protective film (5)