PURPOSE: To fit-up easily a sink to a top plate by welding without generating weld distortion on the top plate, by sticking a wooden plate near a sink fitting opening of the reverse side of the top plate, and thereafter, executing the welding.

CONSTITUTION: A wooden plate 6 is stuck near a sink 2 fitting opening of the reverse side of a metallic top plate 1, also desirably, a water circulating pipe 3 made of copper is provided by abutting it with the marginal face of said opening, the opening margin of the top plate 1 is cooled by making cooling water pass through in the pipe 3, and also the opening margin and the fitting edge of the sink 2 are pinched from the upper and lower parts and are fixed by an upper frame 4 and a lower frame 5 which are made of iron. In this state, by welding a part indicated with an arrow A, it is possible to fit-up perfectly the sink 2 to the top plate 1 by welding without distortion, without performing drawing or step-pressing to the circumference of the opening.