PURPOSE:To bond a silicon atom to oxygen and to improve the sensitivity, life and stability by making a film attached by a biochemical substance into a porous silicon layer. CONSTITUTION:The diffusion area of a source 11 and a drain 12 is disposed in comb-shape from the surface of a single-crystal silicon substrate 10. In the comb-shape area a silicon oxide film 13 and a silicon nitride film 14 are formed so as to form a gate insulating film. The source 11 and the drain 12 are wired and connected by a polycrystal silicon 15. And a polycrystal silicon layer 16 is formed on the silicon nitride layer 14 of a gate oxide film. The biochemical substance such as oxygen is attached to the porous silicon layer 16. When said biochemical substance reacts a specific substance, the potential, on the porous silicon layer 16 develops a change, which produces a change in a current across the source 11 and the drain 12 and is detected.
