An ionizing electrode (3) comprises a flat anode (31a) with a plurality of pointed parts (31b) on the outer edges and a spherical cathode (32a) opposed to the anode (31a). Since the pointed parts (31b) of the anode (31a) do not directly face the cathode (32a), the concentration of the corona discharge is prevented from occurring even if some of the pointed parts (31b) are closer than other pointed parts (31b) to the cathode (32b) due to the machining or mounting errors, resulting in a stabilized corona discharge.