To reduce the influence on measurement by molecules other than hydrogen molecules contained in a sample gas.

In the lower part of the sample cell 3, a detection part 8 using a palladium alloy as photocatalyst is provided. In the vicinity of the center part of the sample cell 3, the photocatalyst layer 4 is formed, and the sample gas introduced to the sample cell 3 from the sample gas inlet part 2, reaches the detector part 8 via the photocatalyst layer 4. On the upper part of the sample cell 3, a UV lamp 14 is provided to have the photocatalyst layer 4 irradiated. By having the sample permeate the photocatalyst 4, the organic compounds CO, and H2S contained in the sample gas are oxidized, decomposed and converted into CO2, H2O, and SO2 and the like.