PURPOSE: To determine concentration of hydrogen in a wide range without changing the temperature setting of nickel diffusion film by arranging a plurality of orifices having thin and small openings of a previously known opening area in parallel in front of an exhaust air device.

CONSTITUTION: In the case of the concentration of hydrogen being from about low to middle degree in a specified determination range, when a vacuum valve 10 is closed, hydrogen which permeates and flows through a nickel diffusion film 3 passes through a vacuum pipe 4, a pressure gauge 5, a vacuum valve 6, presure gauge 8, and a first orifice 9 and then is absorbed by an ion pump 7. At this time, an exhaust air speed of a detector is determined by a conductance of the first orifice 9 and exhausted in a fixed manner. In the case of the concentration of hydrogen being from middle to high degree in a specified determining range, when the vacuum valve 10 is opened, hydrogen permeating and flowing through the nickel diffusion film 3 is exhausted from both orifices 9, 10. Thus, the concentration of hydrogen in a wide range can be determined without changing the temperature of the nickel diffusion film 3.