In a hydrogen sensor (10a, 10b, 10c, 10d), a thin film layer (12) is formed over a substrate (11) and a buffer layer (13) is formed over the thin film layer (12). Further, over the buffer layer (13) is formed a catalyst layer (14) which, by being contacted by hydrogen gas, hydrogenates the thin film layer (12), thereby changing optical reflectance of the thin film layer (12). A constituent of the thin film layer (12) diffusing into the catalyst layer (14) combines with a constituent that has diffused from the buffer layer (13) into the catalyst layer (14), so that oxidation of the catalyst film layer (14) is prevented. Consequently, oxidation of the catalyst layer (14), etc. caused by repetition of hydrogenation of the thin film layer (12) is prevented, and therefore, decrease in hydrogen detection sensitivity of the hydrogen sensor (10a, 10b, 10c, 10d) is restrained.