PURPOSE: To provide an ion-nonsensitive FET by constituting the gate film of a field effect transistor(FET) with a diamond-shaped thin film(DLC).

CONSTITUTION: A DLC made of diamond or made by the coupling of diamond and graphite is used as the gate film of an FET forming a semiconductor ion sensor. When all four valence electrons of carbon atom of diamond are coupled with valence electrons of next carbon atom interminably with no excess electrons left, the so-called diamond type homopolar crystal with a three- dimensional structure is obtained. The amorphous carbon coupled with the hexagonal plate-shaped carbon atom is contained while excess electrons are left, four valence electrons of the carbon atom of a carbon film are almost buried and hardly coupled with oxygen, no oxide is generated, no polarization occurs in a solution, the potential of the film is stabilized, and the DLC can be used as an ion-nonsensitive film.