A twin-hull boat equipped with a pair of transverse hydrofoils 4 and 5 which are respectively secured to the bottom surface of twin-hull structures 3A and 3B adjacent the stem and stern. A pair of auxiliary wings 6A and 6B are installed opposite to each other at predetermined positions on the internal wall surfaces of the twin-hull structures 3A and 3B mutually facing the hull center line and at a predetermined height between the upper water line formed in the course of low-speed cruising and the lower water line formed in the course of high-speed cruising. The twin-hull boat incorporates a postural angle regulating device 15 to properly regulate the postural angle of the auxiliary wings 6A and 6B by causing them to pivot about a horizontal axis.
