An ionization electrode consists of a plate-like positive pole (31a) formed with plural pointed ends (31b) on its outer edge, and a spheric negative pole (32a) opposing a flat surface of the positive pole (31a). Since the pointed ends (31b) of the positive pole (31a) are not in direct face-to-face relation with the negative pole (32a), corona discharge is prevented from concentrating on some of the pointed ends (31b) that are closer to the negative pole (32b) than the rest due to the working errors or mounting errors of the poles. Therefore, the corona discharge occurs in a stable manner.