To provide an off-line teaching device for welding with which the burden on an operator is minimized and the operation of a robot is accurately performed, and a method of manufacturing a large structure or which a stable welding is executed by using the device.

Information on a welding robot, its peripheral equipments, a welding torch mounted on the robot, and a work welded by the robot is stored as a respective three-dimensional model in a storage device of a computer 13 or an external storage device. Stored information is displayed on a display to enable a teaching and simulation of operations of the robot and the peripheral equipments. The computer 13 is provided with a finding means for a section shape which finds a section shape of a fused and bonded metal part laminated on a welded part on the basis of a welding condition set on each path as a path-generation means for a multi-layer welding to be executed by the robot, and a generation means for a next path-position which generates the position of a next path on the basis of a result obtained by the calculation means for the section shape.