PURPOSE: To efficiently perform changing work of a welding jig by making a changing carriage freely reciprocatively between a shunting station and a welding station by the movement of a transfer bar.

CONSTITUTION: Each changeover rail 10b is turned to the longitudinal position and the transfer bar 5 is moved to the welding station A side and the changing carriage 4 is carried in the welding station A and the welding jigs 1 and 1 of automatic welding equipments 2 and 2 at both sides are transferred to the carriage 4. Next, the bar 5 is moved to the front position of the shunting station D and the carriage 4 is returned to the station D and each changeover 10b is turned to the horizontal position and the carriage 4 is sent out onto the minus shunting rails 11 and 11 and the carriage 4 on which the welding jigs 1 and 1 to be used next the loaded is charged from the other side rails 11 and 11 to the station D. The carriage 4 is carried in the station A in the same way and the welding jigs 1 and 1 on the carriage 4 are transferred to the automatic welding equipments 2 and 2 at both sides and the empty carriage 4 is returned to the shunting station D and the carriage 4 is sent out onto the shunting rails 11 and 11 at the minus side or the other side.