PURPOSE: To permit quick discharging treatment of indoor air by increasing the amount of discharging air by a method wherein the title device is constituted so that the flow of discharging air is formed by an exhaust fan in one of exhaust air bypasses of two series, which detour a heat exchanger, while the flow of exhaust air is formed in the other exhaust air bypass by a bypass fan, incorporated into one part of the other series.

CONSTITUTION: An exhaust air passage 7, passing indoor air, and an air supp-lying passage 8, passing outdoor air, are constituted in a main body casing 1 so that they are intersected in a heat exchanger 2 accommodated in the casing 1 so as to permit heat exchange and so as to be independent mutually in all of routes, while an exhausting fan 4, forming the flow of discharging air, is incorporated into the exhaust air passage 7 and an air supplying fan 3 is incorporated into the air supplying passage 8. Two series of exhaust bypasses 13, 14, which detour the heat exchanger 2, are provided in the exhaust air passage 7 so as to be switchable through a damper 18 and the flow of exhaust air is formed in one exhaust air bypass 13 by the exhausting fan 14 while the flow of exhaust air is formed in the other exhaust air bypass 14 by the bypass fan 15, incorporated into one part of the same bypass 14.