To provide a ship bottom antifouling device capable of submerging a rear section of the ship bottom antifouling device into water, facilitating operation to pull up it out of the water, and advancing the ship in a transverse direction.

This ship bottom antifouling device A1 includes a holder body 5 formed into such a shape as to surround the ship bottom, and a float tank 20 which is provided in roughly the center of a bottom surface of the holder body 5 and of which the gravity is smaller than water. The float tank 20 is formed by a tank having a hollow inside, and a sealing structure. Besides, a float force adjusting tank 40 is provided at the front side of the float tank 20, and a float force adjusting tank 60 is provided a the rear side.