PURPOSE: To provide automation and improved performance of a sub-assembly operation by moving back and forth the members placed on a carriage between welding devices provided with torch profiling devices while positioning said members by numerical control thereby subjecting said members to tack welding and normal welding.

CONSTITUTION: An assembling device for an inverted Tee beam is provided with profiling devices 17 provided with welding torches, devices 18 for positioning facing materials and devices 19 for pressing said materials provided to a speed-controllable carriage 11 for mounting facing materials, a device 21 for positioning a web material and a device 20 for holding said material provided with vertical guide roller groups and web clamping devices and respective control panels. The facing materials constituting the inverted Tee beam are mounted on the carriage 11 and are positioned with respect to the web material by numerical control and thereafter said materials are moved back and forth between the welding devices provided with the devices 17, by which the materials are subjected to tack welding and normal welding. The sub-assembly operation is thus automatically executed without the need for a marking operation and jigs for preventing inclination.