PURPOSE: To enhance the sealing property of a sub-intake door by having the sub-intake door opened and closed in response to and in synchronization with the opening and closing of a main intake door, and having the sub-intake door opened outward of an intake unit box.

CONSTITUTION: An outside air introducing port 5 is formed on the top surface of an intake unit box 2 of an air introducing device 1, and a main and a sub inside air introducing ports 6, 7 are respectively formed on its adjoining side faces. A blower is installed inside. The outside and inside air introducing ports 5, 6 are opened and closed by a main door 14 while the sub inside air introducing port 7 is opened and closed by a sub door 15. In this case, an opening and closing relay lever 32 is installed, which possesses an opening and closing drive pin 38 engaged with a driven groove 41 formed on the sub door 15 for opening and closing drive. The lever 32 and the main door 14 are linked and connected with a transmission mechanism 22. The sub door 15 is so set to open outward and rotate from the sub inside air introducing port 7 toward the outside of the unit box 2 in response to the rotation of the main door 14.