PURPOSE: To improve the permeating and capturing efficiency for gaseous hydrogen by providing a tubular gaseous hydrogen permeable membrane of which both ends are opened in oil, a casing forming a gaseous hydrogen capturing chamber by contg. said membrane, etc.

CONSTITUTION: The casing 2 of a detector 1 for gaseous hydrogen in oil forms a gaseous hydrogen capturing chamber 3 and is provided with a mounting flange part 3. A tubular gaseous hydrogen permeable membrane 9 is provided in the part 4 by penetrating both ends 10, 11 thereof through said part. The membrane 9 is so disposed that both ends 10, 11 thereof open in the oil. When a heater 12 is heated, the oil in the membrane 9 is heated and rises to eject through a pipe end 11. Therefore, the oil flow such as the one shown by the arrow is produced, whereby the gaseous hydrogen in the oil is permeated and captured successively. Since the oil and the gas in the oil are expanded by the heating, the sepn. of the hydrogen is made easy and the efficiency for capturing the same is improved.