An anion generating device includes an outer housing formed with air inlet and outlets as well as a heat exchanging space, a fan unit disposed in the heat exchanging space, an anion generator operable so as to generate anions near a discharge electrode unit thereof disposed between the air outlet of the outer housing and an air outlet of the fan unit, and a power control unit disposed in a receiving space in the outer housing for supplying electrical power to each of the fan unit and the anion generator by means of electrical energy converted from solar power by a solar power collecting plate mounted on a heat conductive top wall of the outer housing. The anions are impelled by air expelled by the fan unit to move outwardly of the outer housing via the air outlet of the outer housing.