PURPOSE: To reduce trunk space while to prevent noise and overload of blower and to provide air cleaning capacity by constructing an inner air suction duct with main and branch ducts exchangeable through a flow path exchange door while arranging an air-cleaner in the branch duct.

CONSTITUTION: The confluence port 47 of main duct 43 is closed by an exchange door 5 to form an inner air flow path passing through confluence path 46 in a branch duct 44, branch duct 44 and air-cleaner 6. Consequently the air in the cabin is cleaned. When a blower 24 is driven hard, an exchange door 5 is exchanged to the position shown by dashed lines through an actuator 55 thereby the air is not flowed to the branch path 44 but the flow path is formed by main duct 43. Consequently the air will circulate without flow resistance thus to prevent noise and overload of blower 24.