PURPOSE: To promote responsiveness of a polarographic type oxygen sensor, by attaching a porous insulating material to the periphery of a cathode's top section where the cathode is brought into contact with a diaphragm of the oxygen sensor.

CONSTITUTION: A cathode 4 of a polarographic type oxygen sensor 10 is supported with an insulation holding material 30 so that the peripheral side of the cathode 4 may be exposed up to a prescribed height (for example, 1mm) above its end where the cathode 4 is brought into contact with a diaphragm 8. An annular porous insulation holding material 40 is attached to the exposed peripheral side of the cathode 4. Thus, since the migration of ions of an electrolytic solution 6 into the top of the cathode 4 is promoted through voids in the porous part 40, the responsiveness of the oxygen sensor is improved.