To provide a fin control device for suppressing cavitation by detecting the occurrence of cavitation in a fin.

A fin control device for controlling the inclined turning angle of a fin 1 used in the water close to a water surface includes a cavitation detecting means 22 for detecting the occurrence of cavitation in the vicinity of the outer plate 20 of the fin 1 and a control means 23 for controlling, when the occurrence of cavitation is detected by the cavitation detecting means 22 and if a specified fin angle exceeds a fin angle at the time of the cavitation occurrence, the specified fin angle to be maintained equal to the fin angle at the time of the cavitation occurrence.