PURPOSE: To simply and exactly divide the air sent from a fan into a propelling air and an air for buoyance by providing partition plates so that the partion plates form a sector by a central point and an outer peripheral circular arc surface of a fan duct.

CONSTITUTION: With an air current sent by a fan 2 installed in a fan duct 1, a part of the air current is introduced as a propelling air current (A) into the rear of a hull 3, and the remaining air is introduced as an air current (B) for buoyance into a skirt 5 by means of partition plates 4. In this case, for the partion plates 4, two plates are installed in the axis direction of the fan so that the plates form a sector by a central point and an outer peripheral circular arc surface, and extended from the central direction of the fan 2 to the fan duct 1 and fixed at the leeward direction. A ratio of the air volume for buoyancy to the whole blasted air volume of the fan 2 can thus be exactly set.