PURPOSE: To make it possible to detect the temperature of suction air promptly and accurately by means of a temperature sensor by introducing hot air sucked up from a suction port directly from a bypass passage and feeding said air to said temperature sensor without reducing the humidity of the suction air.

CONSTITUTION: A temperature sensor 11 is installed to a bypass passage 4. When there exists an obstacle 12, such as wall near a hot air blowoff port 10, the hot air is collided against the obstacle 12 where the direction of the hot air is changed so that it may be absorbed into a main body case 1 from an air suction port 2. A part of the suction air passes through the bypass passage 4 where the temperature of the suction air is detected by the temperature sensor 11. When the suction temperature exceeds a preset temperature, a heating element 9 or the heating element 9 and a blower 5 come to a halt. It is, therefore, possible to prevent the heating of a dust filter 3 and the blower 5.