an oxygen electrode comprising a plastic sheet , two or three electrode patterns formed on the plastic sheet , a paraffin layer formed on the peripheral portion of the plastic sheet for delineating a sensitive area , a carrier sheet impregnated with an electrolyte and placed on the central portion or the sensitive area of the plastic sheet , and a gas permeable membrane covering the carrier sheet and the paraffin layer . a biosensor may be also provided by further providing a microorganism or enzyme - immobilized layer on the gas permeable membrane of the above oxygen electrode .