PURPOSE: To aim at the optimum positioning of a mast at the time of reaching in the structure of centerboard installation part of a sailing board by expanding a mast rail on a center board case and more front than the center board case, and projecting an upper end operating part of the center board upward of the mast rail side for arrangement.

CONSTITUTION: A channel 20 of a center board case 2 is expanded in the central part of the cross direction of a board 1, and a mast rail 4 is expanded in front more than the channel 20, and a joint 61 of a base of a mast 6 is engaged in the rail 4 to be movable in the longitudinal direction along the rail 4. A horizontal shaft 30 of the center board 3 is turnably supported by both side walls of the channel 20, and an upper end operating part 32 of the center board 3 is projected upward in both sides of the mast rail 4 for arrangement. With this structure, the center board 3 can be rotated with the operation of the operating part 32, and the mast can be positioned at the optimum position at the time of reaching.