PURPOSE: To restrain wind from temperature rise and enable energy to be recovered by providing a booster fan for accelerating wind flowing out of a swimming chamber and blowing up it from the lower portion of the other swimming chamber to prevent wind passing through a fan and the booster fan from circulating.

CONSTITUTION: Air sucked by a fan 1 passes through a wind path 5 to consume energy while giving buoyancy to a swimmer 4 in a swimming chamber 3. The air again passes by a wind path 5' through a booster fan 2, giving buoyancy to a swimmer 4'. Since the same things can be repeated any times, the number of repetition stages corresponding to the necessary number of swimmers can be determined. Thus since this system is an open one, heat is not accumulated in the air, so that the temperature rise of wind is few and even small capacity of the booster fan will do.