Light from light source means (a wavelength-variable laser) is applied to a surface of a hydrogen absorbing thin metal film of a hydrogen detecting surface plasmon resonator including a surface plasmon resonance enhancement structure formed by providing in the thin film an array of periodic holes having a shape that is not 90-degree rotational symmetric in the plane of the film surface, and transmitted light is detected with light detecting means (a photometer). Hydrogen is detected on the basis of a change in light transmission frequency characteristic caused by hydrogen absorption in the hydrogen detecting surface plasmon resonator. Optical hydrogen detection that is highly safe and unaffected by variations in the amount of light from the light source and stray light can be achieved.
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