PURPOSE: To contrive intake of fresh air under improving solar heat utilizing efficiency by a method wherein ventilation is permitted under effecting heat exchange between return air and atmosphere in a circulating duct from a room to a solar heat collector.

CONSTITUTION: Warm-air, generated by the solar heat collector 2, flows into the room 9 to heat it from a ventilating port 23 through a supply duct 4 by opening dampers D1, D2 while the warm-air flows into a heat-accumulating tank 10 from a branch duct 24 to accumulate heat. The return air from the room 9 flows into an exhaust path 16 and the return air from the heat accumulating tank 3 flows also into the exhaust path 16 through an air sending path 44 in the same manner. When an amount regulating valve 35 is opened slightly in this case, a part of the air flows into a suction path 17 from the ventilating path 33 and the balance of the air is discharged from the ventilating port 29 through the exhaust path 16. In case of discharging the air, heat is accumulated and moisture is also accumulated in a heat exchanging unit 7 and atmosphere, flows into the suction path 17 through the ventilating port 30, effects total heat exchange and, thereafter, the atmosphere is circulated to the solar heat collector 2 through an air inlet port 4.