PURPOSE: To perform simple and precise immunoassay, by measuring the membrane resistance or passing current between both surfaces of a lipid double layer membrane having the antibody or antigen to a substance to be examined immobilized thereon.

CONSTITUTION: A silver electrode 2 having an Ag/AgCl electrode surface 3 formed to the upper surface thereof, a ceramic spacer 4, a ceramic filter 6 having a lipid double layer membrane 7, which has a monoclonal anti-human AFP antibody immobilized thereon, formed to the upper surface thereof and a silver electrode 8 having an Ag/AgCl electrode surface 9 formed to the inner surface thereof are successively laminated to the interior of the through-hole provided to the central part of a support from the bottom part side. The hollow part on the silver electrode 2 is filled with a physiological saline solution 5 so as to penetrate in the voids of the ceramic filter 6 to be brought into contact with the under surface of the lipid double layer membrane 7. Further, measuring terminals 10, 10 are connected to the silver electrodes 3, 8. Measurement is performed by dripping a specimen solution 11 and a complement on the lipid double membrane layer 7 to measure the change of membrane resistance by the measuring terminals 10, 10.