PURPOSE: To provide a hydrofoil for a catamaran unsubject to damage both the used time and non-used time with no need to be folded, simple in structure, large in buoyancy so as to be able to float at low speed and with small output, and as a result, small in fuel consumption.

CONSTITUTION: A catamaran 1 is provided with an almost horizontal hydrofoil 12 fitted between the twin hulls 2 there of in such a way as to be oscillated around a horizontal shaft 11. Another hydrofoil device 12 is formed of two almost horizontal hydrofoils 12 disposed being spaced vertically between the twin hulls 2, and vertical beams for connecting these hydrofoils to each other. Both ends of the hydrofoils 12 are respectively fitted to the inside of the double- barrel hulls 2 in such a way as to be oscillated around the horizontal shafts 11, and both ends of the vertical beams are pivotally fixed to two hydrofoils 12.