PURPOSE: To prevent the damage of a connecting device by turning ON/OFF of the magnetic force of a magnet, detecting by a switch the connection, detachment, and fixation to a prescribed position of the supporting trucks of a looper, making the turning ON of the magnetic force only at the time of a necessity.

CONSTITUTION: When truck 4' and loop car 1 connected together approach a truck 4 fixed on a ground pole 8, it is detected by the connection proximity switch of the truck 4', the commands of the turning ON of a magnetic force to a connection magnet 14, the turning OFF of the magnetic force to a fixed magnet 11, and the turning OFF of the power supply of a fixed proximity switch, are outputted. Consequently, the truck 4' is connected to the truck 4, and also, the truck 4 is detached from the ground pole 8. The trucks 4, 4' and the loop car 1 are moved as one body to a deflector roll 3 side. When they are moved on the opposite side, the order is reversed to operate them, and the trucks 4, 4', and the loop car 1 are detached. In this way, since the magnetic force is turned ON by the magnet only at the time of a necessity, at the time of a detachment, an electric current is broken, and the damage of a connecting device can be prevented.