in embodiment 1 except that the concentration part 300 and the hydrogen gas sensor element 500 are separately arranged by using palladium which absorbs only hydrogen as the hydrogen absorber 5 formed in the thin film 10 of the substrate 1 the absorbed hydrogen is discharged by heating to concentrate the hydrogen gas in the micro chamber 100 therefore since the palladium film as the hydrogen absorber 5 maintains a high selectivity to hydrogen gas the hydrogen gas sensor with high selectivity to hydrogen gas can be provided not necessarily requiring the hydrogen gas sensor element 500 formed at the side of the cover 2 to have selectivity to hydrogen gas in fig8 as described above a diagram of an embodiment of the configuration in the hydrogen gas sensor in the present invention is illustrated in the case described above a hydrogen absorbing material such as a palladium film is used as the hydrogen sensitive layer 6 formed in the thin film 11 and heat generation caused by absorbing or adsorbing hydrogen is utilized a platinum catalyst in which platinum fine powders are mixed into alumina can also be used as the hydrogen sensitive layer 6 in this case the hydrogen gas sensor element 500 can operate as a contact combustion type hydrogen gas sensor it can also be used as the hydrogen gas.