PURPOSE: To freely adjust the attachment and detachment force of a mast in a slide member that can freely be moved and stuck in the forward and backward directions of a boat body by hooking a pin energized by a spring in an upper mast mounting hole and providing a knob that adjusts the spring force.

CONSTITUTION: In a slide member as the mast support device that can freely be moved in the forward and backward directions of a boat body and can be stuck at an arbitrary position, a pin 91 energized by a spring 100 is fit in an upper mast mounting hole 79. The spring 100 is hooked between the pin 91 and a spring receiver 102. The spring receiver 102 is screwed in a spring 106 stuck to an adjustment knob 104 and engages the notch 103 with a guide protruded from a joint 78a and then is held so as not to rotate. The spring receiver 102 is receded by turning the adjustment knob 104. As a result, the attachment and detachment force of the mast can freely be adjusted by varying compression force against the pin 91 of the coil spring 100.