PURPOSE: To enable rapid measurement of various components, by laminating an org. or inorg. catalyst layer on the diaphragm of an oxygen pressure equilibrium oxygen monitor.

CONSTITUTION: A comparison electrode 1 controls the potentials of cathodes 5 (5aW5c) and anodes 6 (6aW6c). A high-molecular electrolyte is used as an electrolyte 2 in common and an electrical insulating pipe 3 having a closed leading end comprising plastics and ceramics has holes 8aW8c communicated with the electrolyte. An oxygen permeable membrane tube 4 comprises a Teflon tube and partitioned into chambers by O-rings 9. The film 7 (7aW7c) of each catalyst layer through which reactive substance and a reaction product can be freely diffused is selected from arbitrary combinations corresponding to a measuring object. For example, if the combination of enzyme films each having oxidation enzymes immobilized thereby alone or compositely is used, the concn. of each component can be instantaneously calculated from the change in the output current of a unit current by one sensor.