PURPOSE: To reduce the installation space by directing a corona discharge electrode to the wave direction so that the just opposite surface of thereof is a wall of an enclosing box, and providing air blow-off ports at the places where the discharge electrode is not seen directly from the outside and an air supply means at the back positions of lines connecting the discharge electrode and earth electrodes.

CONSTITUTION: The wave 6 is saved from the emission fog of a corona discharge by directing a corona discharge electrode 1 to the wave 6 direction so that the just opposite surface of the discharge electrode 1 is a wall 10 of an enclosing box 5, and providing air blow-off ports 8a, 8b directed to the wave 6 at the positions where the discharge electrode 1 is bot seen directly from the outside. An air supply means 9 directed to the air blow-off ports 8a, 8b is provided at the back positions of lines connecting the discharge electrode 1 and earth electrodes 3 to prevent that the ion generated in the discharge electrode 1 is drawn by the earth electrodes 3a, 3b. The generated ion is used effectively by forming a flow for driving out the ion from the enclosing box 5 through the air blow-off ports 8a, 8b with the air flow. The installation space is thus reduced.