PURPOSE: To simplify the mast supporting structure and to ease assembling or disassembling of a catamaran, in which a pair of float body are detachably connected together by beams, by installing a mast holder as one body to a member that connects both beams together.

CONSTITUTION: A pair of float body 1 consist of bag bodies, made of waterproof canvas material like rubber, whose insides are filled with compressed air to maintain the shape. Each float body 1 is connected with each other by making beams 2 and 3 to penetrate locking pieces 13 provided at both fore and aft parts and outer and inner sides of each float body 1 in such a manner that the beams 2 and 3 can be freely attached/detached to the pieces 13. Sleeves 41 provided at both ends of a connecting rod 42 are made to penetrate the beams 2 and 3 which are connected together by a connecting member 4. A mast holder 43 is fixed to the crossing point between the forward sleeve 41 and the rod 42. A mast 6 is inserted into the holder 43, underneath of which a center board 9 is held via a rib 90. A canvas sheet 14 is spread out between the float bodies 1.