This air purifying device is provided with: a housing (50) having an air passage (52) through which the air to be disinfected (51) flows from upstream to downstream; a spray device (53) which sprays a bactericide solution into the air to be disinfected (51) which is flowing through the air passage (52); a reservoir (54) for receiving the bactericide solution falling from the air passage (52); a solution supply system (55) for supplying to the spray device (53) the bactericide solution of the reservoir (54), a drug supply device (56) for supplying the bactericide solution to a circulation system, a diluent system (571) for supplying a diluent for diluting the bactericide solution, and a bactericide solution circulation system formed by the spray device (53), the air passage (52) of the housing (50), the reservoir (54) and the solution supply system (55). The circulation system has a bactericide inflow site where undiluted bactericide solution flows into the circulation system from the drug supply device (56), and, at a different position, a diluent inflow site where the diluent flows into the circulation system from the diluent system (571). At the bactericide inflow site, the circulation system has a bactericide unused liquid region formed by bactericide solution with higher bactericide concentration than that of the other bactericide solution in the circulation system. After killing the bacteria in the bactericide unused liquid region which are still alive in the bactericide solution flowing into the bactericide inflow site, the bactericide solution is passed through the solution supply system (55) and is again sprayed from the spray device (53) into the air to be disinfected.