PURPOSE:To enable positioning in good precision even though a vessel is deformed by arranging a positioning fitting at the bottom face of molten metal vessel and arranging the device, which supports/fixes the positioning arm and working machine engaging to the fitting free to elevate. CONSTITUTION:A molten metal vessel is tilted, making a bottom iron shell vertical. A mounting device 31 is run up to the prescribed position and is stopped. Sucessively, an elevating cylinder 36 of the mounting device 31 is actuated, engaging positioning arms 46-48 of a mounting frame 40 with positioning fittings 20-22. The elevating cylinder 36 is actuated until a support bracket 41 of the mounting frame 40 and a support pin of the mounting device 31 are detached. When the tip face of a lower positioning arm 48 is brought in contact with Z axis upper surface of a positioning fitting 22, by actuating the locking mechanism, the vessel is fixed in Z axis direction with retracting the cylinder.