Abstract: A field effect transistor for use as an ion sensor has a P-type silicon substrate on which are formed a source region and a drain region. An N-type isolation diffusion layer is formed on the outer peripheral surface of the silicon substrate and this diffusion layer is surrounded by an insulation layer. According to this arrangement, even when the potential of the electrolyte has been raised to a level which is positive with respect to the silicon substrate, an electrical isolation is established by the reverse dielectric strength exhibited by the P-N junction.