An FET-based microsensor using a proton conductor for detecting a small concentration of H 2 in air at room temperature has been fabricated by attaching an antimonic-acid thick film (≈ 20 μm thick) and a sensing Pt electrode to an ISFET device. Under a constant drain current (I D ) at 30°C, the output voltage (V GS ) between the sensing electrode and the source is almost linearly correlated with the logarithm of H 2 concentration in the range 4−5000 ppm. The slope of the correlation, ≈ − 120 mV/decade, is comparable to that for the non-FET type solid-state potentiometric H 2 sensor previously reported. The 90% response time to 5000 ppm H 2 is as short as ≈ 5 s. The H 2 -sensing mechanism of the present microsensor is briefly discussed.