A rotary shaft is inserted to a tank through a hole formed in a wall of the tank. A casing is provided outside the tank, and the rotary shaft is supported by a bearing arranged in the casing. The casing is watertightly connected to the wall including the hole through a flexible joint. Seal members are arranged in the casing to seal the clearance between the rotary shaft and the casing. A sleeve is fitted on the rotary shaft so that the sleeve is watertightly in contact with at least one of the seal members, and the sleeve can be moved along the rotary shaft to change the seal member in contact with the sleeve. The bearing and the seal member are not affected by the deformation of the tank since the flexible joint connects the tank and the casing, and the rotary shaft can rotate smoothly while securing high sealing quality. The alignment of the axes of the bearing and the hole in the tank can be easy since the bearing is provided outside of the tank, and thus assembly and maintenance of the structure are easy. The tank can have relatively low strength since the tank has not to support the rotary shaft with the bearing; thus the structure can be compact and the cost can be reduced.