In the phosphate immersion treatment of a steel sheet structure such as an automobile body, a method is provided wherein the bottom of a treatment tank is flat. An object to be treated such as an automobile body is submerged in the treatment solution. The object to be treated is inclined upward and downward at least once to remove oil, reaction cases, and air collected inside the object. An apparatus for performing phosphate immersion treatment of the object is also provided wherein the treatment solution is sprayed toward the feeding side of the treatment tank to cause small waves, so that formation of a border line mark on the surface of the object due to temporary interruption of the feeding operation may be prevented. An undulation is formed in a conveyor arranged above the treatment tank, or a pair of drop lifters which are individually operable are mounted to the conveyor, so that the object may be inclined upward and downward.