PROBLEM TO BE SOLVED: To provide a thin film-type hydrogen gas sensor capable of detecting a hydrogen concentration with a simple structure.SOLUTION: In a thin film-type hydrogen gas sensor having a sensitive part composed of a platinum thin film formed on a top face of an insulator substrate, a film thickness of the platinum thin film is 40 nm or less, and a lower layer of the platinum thin film is provided with a metal thin film. An undulation of 10 nm or more is formed on this metal thin film. In particular, the lower layer of the metal thin film is provided with an insulation film whose surface is formed with the undulation of 10 nm or more, and the undulation of 10 nm or more is formed on the surface of the metal thin film by forming the metal thin film on the top face of this insulation film. In addition, a bridge circuit is formed by the platinum thin film of the sensing part and three resistors having a resistance value close to the resistance value of the platinum thin film of this sensing part, and this bridge circuit is driven by AC voltage, and a differential between an AC output signal and a reference signal is taken.
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