To improve the dust-collecting efficiency and maintain the dust- collecting efficiency, namely make the cleaning period of the dust-collecting section longer for an electric dust collector set in a driveway tunnel, an underground parking lot and the like, and treating air containing dust.

An electric dust collector is provided with a first stage dust collecting section 4 and a second stage dust collecting section 5, and respective stages are connected with respective high voltage power sources V1 and V2, and the applied voltage is increased toward the outflow side (V1<V2). The volume of dust collected quickly is the first stage dust-collecting section 4 is reduced, and the dust corresponding to the reduced volume is collected in the second stage dust-collecting section 5 to provide an electric dust collector for collecting the dust uniformly in respective dust collecting sections and preventing the release and re-scattering the dust accumulated on a dust collecting plate 111. Further, the same voltage is applied to the respective stages from the same high-voltage power source and the plate intervals of the first stage dust- collecting section and the second stage dust-collecting section are narrowed successively from the inflow side toward the outflow side to produce the same effect.