PURPOSE: To elevate the integration without degrading characteristics such as sensitivity and S/N ratio by arranging one source in common to a plurality of ion selection sensitive film to increase the integration of an ISFET with a reduced ratio of the source area to the total area.

CONSTITUTION: A common source 2 and drains 11W14 are formed on a P type Si substrate and an SiO2 film 3 and an Si3N4 film 4 are attached thereon to make an insulator, on which Na+ selection sensitive film 31, a K+ selection sensitive film 32, a Co++ selection sensitive film 33 and a Cl- selection sensitive film 34 are formed. In other words, the source 3 is arranged in common to reduce the ratio thereof 2 to the total area thereby elevating the sensitivity with an higher area ratio of the sensitive films 31W34. In addition, drains 11W14 of individual sesitive sections are arranged between a part of the common source 2 to improve the shield effect and the S/N ratio.