PURPOSE: To prevent the floating of an object being treated in the surface treatment process in which the object is dipped in a treatment tank by a method in which a treating liquid is sucked up from the bottom of the tank and discharged from both sides for circulation.

CONSTITUTION: In subjecting an object 7 to be treated, e.g., an automobile body, etc., to a surface treatment using a treating liquid 5 of a phosphate, etc., in a treatment tank 1 or a water-washing treatment process, the object 7 is hung down from a hanger 8 and dipped in the treating liquid 5 through a conveyer 6. In this case, the object 7 with cavities is often floated and separated from the hanger 8. Thus, the treating liquid 5 is sucked up from the suction port 9 provided at the bottom of the entrance part of the tank 2 of a treatment tank by means a pump 11 and then returned through port 10 provided in the side of the tank 1 from conduits 12 and 13 into the tank 1, whereupon the object 7 is pulled down by the treating liquid being sucked and circulated with stirring. Thus, the surface treatment and water-washing treatment of the object 7 to be treated can be uniformly attained.