To easily lift or lower a boat by connecting a pair of right and left rails and providing a rotary support structure between outer and inner rails having a telescopic bouble structure and fixing a slidable mounting base to a pier or the like to lift or lower the boat.

A structure for connecting a piar of right and left rails is provided. A rotary support structure for supporting the inner rail is provided in the outer rail having a telescopic double structure. A boat slider 1 is fixed to a pier and a strap of a winch 5 is connected to the boat 2 and the boat is wound up to an appropriate position of the mounting base. In this case, the mounting base is fixed by a lock pin 3. When the boat 2 is wound up to an appropriate position, the lock pin 3 is unlocked and the boat is drawn up together with the mounting base. When the boat is to be lowered, the mounting base is prevented from getting out of the rails by a lock plate 4. The boat slider may be fitted to a boat trailer. Travelling is made to be possible in the mounted state. The mounting base is made to be a slide type. By this constitution, large crane equipment, etc., is not necessary and improvement of safety and savings of time and cost can be secured.