PURPOSE: To improve an SN ratio and to obtain a sensor having high sensitivity by using a monomolecular (LB) film having a group which reacts with specific ions for an ion sensitive film.

CONSTITUTION: The single or plural layers of the LB (monomolecular) films 3 which are formed by a Langmuri-Blodgette's process and are closely arranged with the molecules of cytochrome C are used as the ion sensitive film. The cytochrome C induces an antigen-antibody reaction selectively with γ-globulin 10, i.e., said cytochrome C has ion selectivity. The films 3 can be controlled to several 10 film thickness. The ion sensor constituted in such a manner has FET structure and the drain current thereof is controlled by a potential difference between a reference electrode 7 and the LB films 3 corresponding to a gate electrode. The cytochrome C is oriented to a high degree by using the LB films 3, by which the γ-globulin can be efficiently attached thereto an therefore, the electric field to be applied to the gate is eventually increased. The sensor having high sensitivity is thus obtd.