To solve a problem that the lightening of a small-sized planning boat can not be achieved when projecting parts are thickened to increase their rigidity as the projecting parts projecting from right and left side parts have low rigidity in a conventional seat for the small-sized planning boat having a structure with the projecting parts forwardly projecting from the right and left side parts of a bottom plate.

The bottom plate 50 of the seat for the small planning boat has a right side part 31 extended from a right end of a seat part 30 to a lower part with respect to the seat part 30, a left side part 32 extended form a left end of the seat part 30 to a lower part with respect to the seat part 30, and a reinforcement part 40. The right side part 31 has a right projecting part 31a forwardly projecting with respect to the seat part 30, and the left side part 32 has a left projecting part 31 forwardly projecting with respect to the seat part 30. The reinforcement part 40 is hung between the right projecting part 31a and the left projecting part 32a.