To provide an auxiliary carriage of a one side welding apparatus having a plurality of backing devices arranged thereon, which auxiliary carriage can surely support a panel portion between the backing devices at its middle portion irrespective of the number of the backing devices.

When backing carriages 4, 5 are moved so as to coincide with the position of a welding line, the auxiliary carriage 6 moves by (1/2)×L in the same direction of the backing carriage 4 during the movement L of the left backing carriage 4, because the gear 3 of the auxiliary carriage 6 engages with the racks 1, 2 of the backing carriages 4, 5. Therefore, the auxiliary carriage 6 always moves to the middle position between the backing carriages 4, 5, and its roller 13 always supports the panel from below at the middle position between the backing carriages 4, 5. As a result, the roller 13 can effectively prevent the hanging-down of the panel due to its weight.