PURPOSE: To excellently prevent the flowing of dust into an electrostatic precipitator by providing a dust separating gallery having the impellers vertically arranged in multiple stages and having respective variable inclinations on the inlet side of the precipitator.

CONSTITUTION: A bypass tunnel 3 communicating with the main tunnel 1 of a travel road is provided in the tunnel for an automobile road, and the electrostatic precipitator 4 and a blower 5 are set in the bypass tunnel 3 to clean the air in the tunnel. The dust separating gallery 6 having impellers 6a vertically arranged in multiple stages and having respective variable inclinations is provided on the inlet side of the precipitator 4. As a result, the dust generated in the tunnel can be separated and removed on this side of the precipitator, hence the amt. of dust to be collected by the precipitator can be reduced, the deposition of dust in the precipitator can be prevented, maintenance services for the precipitator are reduced and troubles in the precipitator due to an antifreezing agent for the road can be surely obviated.