PURPOSE: To prevent an air-conditioning system after a blower from being influenced by the inhibition of intake due to the clogging of an outside air filter and the like at the time of operating in an outside air mode.

CONSTITUTION: A dust collecting device for a vehicle is provided with an outside air intake 3, an air box 1, an obliquely disposed filter 6, and an air case 12 having a by-pass aperture 11. The by-pass aperture 11 is provided with an opening/closing door for opening the by-pass aperture 11 by negative pressure when the air case 12 becomes negative pressure, and a duct 5 is provided with an outside air suction port 14, an inside air intake 15, and an inside-outside air switching door 16 allowing the outside air intake 14 and inside air intake 15 to be opened/closed alternately. The figure 7 is a blower. When intake is inhibited due to the clogging of the filter, the icing of the filter surface, or the like at the time of an outside air mode, filter back wash becomes negative in pressure, and the by-pass door is opened by the negative pressure to open the aperture 11. At the same time, a switch is operated to move the inside- outside air switching door 16 so as to close the outside air intake 14 and to open the inside air intake 15. The outside air mode is thereby switched to an inside air mode to maintain the circulating air quantity.