PURPOSE: To reduce an adhesion of dust by a method wherein each of two air intake ports is located in two surfaces opposing to each other of a box body to which each of the surfaces is contacted around a corner part and a shielding plate having a half of the area of an air intake port is movably ar ranged.

CONSTITUTION: Two right and left air intake ports are arranged at two right and left locations on side surface of a box body 1 in such a way as two surfaces are opened around a corner part of a full heat exchanger element 2. A spacing 9 between a flange 7 and the box body 1 is fixed with a shielding plate 8 having a half of the area of the air intake port. At this time, indoor air is flowed as shown at an air flow 10, into the full heat exchanging element 2 through a returning port 3, discharged out of a discharging port 5 through a room, an ambient air enters through an outer air intake port 6 into the element 2 and then flows out into the room through an air supplying port 4. Two gases flow in the element 2 in rectangular directions and are heat exchanged. A shielding plate 8 is moved and set at the opposite side of the intake port, resulting in that the air is flowed in an opposite direction as shown at an air flow 11 and the dusts adhered within the element 2 may easily be removed.