To increase the number of negative ions generated per unit time, and the number of negative ions supplied to a subject space per unit time.

An opening 15 in a cylindrical ground electrode 13 is formed in such a way that a cross sectional surface at a second position 19 on the downstream side is smaller than a cross sectional surface at a first position 18 on the upstream side, a discharge electrode 12 is disposed in the opening 15 to be aligned with the second position 19 or positioned on the downstream side of the second position 19, so a high speed gas flow is negatively ionized around the discharge electrode 12 to increase the number of negative ions generated per unit time. The discharge electrode 12 is composed of plural needle electrodes arranged in a diametric direction in the opening 15 to increase the number of negative ions generated per unit time.