PURPOSE: To improve directional stability or tacking property suitable to a using purpose by making a change in distortion property at the position of the longitudinal axis of a fin blade.

CONSTITUTION: The fin of a surfboard is constructed from fiber-reinforced plastic having a plurality of fibers 5 which extend in a preferred fiber direction with the majority of the fibers lying in this direction, and the preferred direction extends at an acute angle α to the longitudinal axis 4 of profile of the fin blade 2. This allows the distortion property at the vertical cross-section of the longitudinal axis 4 to change accordance to the position of the longitudinal axis, and then the angle of incidence δ of the fin blade 2 against the flow direction of fluid can be changed. As a result, it is possible to prevent 'spin out', and to improve directional stability or tacking property.