PURPOSE: To form an Al-diffusion layer while partially ununiformizing the thickness thereof, in applying Al pack treatment to metal parts in a treatig tank, by separating treatment into two stages to coat a part of the metal parts in the front stage.

CONSTITUTION: Metal parts, for example, a tubine 1 is placed on the receiving table in a treating tank 5. In the next step, a shield case 6 provided in an up-and-down movable state is moved downwardly and the opening part 11 thereof is contacted with the turbine 11 in an almost hermetical state to be inlaid therewith and the parts other than the blade leading end of the turbine 1 are covered hermetically. In this state, a lid 15 is opened to fill the case 6 with a treating powder 13 having an Al-powder mixed therein and, after the lid 15 is closed, the turbine 1 is held under heating in an atmosphere such as H2 in this closed state to be subjected to Al-pack treatment. By this method, an Al-layer is formed to the surface other than the blade leading end part of the turbine 1 in a uniform thickness. In the next step, the case 6 is upwardly moved to widen the powder 13 in the tank 5 and the powder 13 is brought into contact with the entire surface of the turbine to again perform Al-pack treatment and an Al-layer is formed to the blade leading end part of the turbine similarily as mentioned above while the thickness of the Al layer on the other part is increased.