PURPOSE: To increase the sensitivity of a hydrogen sensor by making use of the decrease in the resistance value of tungsten oxide when gaseous hydrogen is absorbed in the tungsten oxide.

CONSTITUTION: Two electrodes 2, 3 are formed by vapor deposition of Pt, Pd, Ti, etc. on a substrate. An amorphous WO3 film 4 is provided on the electrodes 2, 3 across said electrodes to 4.0W6.0g/cm3 packing density. A Pt or Pd layer 5 is provided on a WO3 film 4 to constitute a hydrogen sensor. When hydrogen is grought into contact with the sensor, the hydrogen is absorbed into the film 4 by the catalytic effect of the Pt layer 5. The resistance value of the film 4 is consequently decreased and the resistance between both electrodes 2 and 3 is decreased. The hydrogen is thus absorbed more into the WO3 film and the resistance is lower as the concn. thereof is higher. The hydrogen is detected with high sensitivity.