PURPOSE: To move further fix a mast to an optional position, by providing with a mast bottom part to a step guide rail having plural teeth on the top surface of a sailing board main unit, through a detachable mast step and a lock device.

CONSTITUTION: A step guide rail 7, having plural teeth 9, is fixed to the top surface of a sailing board main unit, and a mast step 10 is longitudinally movably mounted to said step guide rail 7. Lock parts 33 of doglegged shape are longitudinally provided to the mast step 10 through springs 28, and constituted such that the mast step 10 can be moved further fixed by depressing an upper end part of the lock part and disengaging the mast step 10 from the teeth 9 of the step guide rail 7. The bottom part of a mast 15 is connected to the mast step 10 through a universal joint or the like. The mast can be easily moved further fixed to an optional position of a boat.