To restrain radioactive waste liquid fluidized by the rotation of agitation blades, from inducing the vibration of an internal structure by providing a cylindrical body submerged under the liquid level in a vessel body and vertically extended surrounding the agitation blades.

A cylindrical body 10 submerged under the liquid level of radioactive waste liquid and vertically extended surrounding agitation blades 5 is provided in the center position in a vessel body 1, being supported to the internal wall of the vessel body 1 by supports 11. The lower end of the cylindrical body is throttlingly formed to reduce its bore diameter gradually downward to form a nozzle hole 12. When the agitation blades 5 are rotated to form downflow to the radioactive waste liquid in the vessel body to make it flow, the downflow flows down being guided by the cylindrical body 10 to reach a bottom part and then turns around to become flow that ascends around the cylindrical body 10. The flow of the radioactive waste liquid 3 from the lateral direction weak in strength is therefore prevented from colliding directly with an internal structure such as a waste liquid feed pipe 2 and a waste liquid drain pipe 4.