PURPOSE: To accelerate the sedimentation velocity, by giving the magnetic field to the sludge in treating solution in the metal surface treating vessel and then, absorbing and growing the sludge each other maintaining magnetic moment.

CONSTITUTION: The metal surface treating vessel 1 used for phosphate coat chemical forming treatment, is shaped in ship bottom shape and plural sludge reservoirs 2 are provided the bottom part and the valve for takeout 3 is provided to said reservoir 2. The treating solution 4 mainly composed of phosphate is put in the vessel 1 and a large quantity of insoluble phosphate compound produced by the reaction of metal, that is, the sludge 7, is contained. One or plural magnetism generators 6 composed of permanent magnet or electromagnet, are provided to the side face of the vessel 1 and magnetic field is given to the solution 4 by the generator 6. Hereby, the sludge 7 is retained a magnetic moment and is grown each other by absorbing action and then, is rapidly settled down.