PURPOSE: To execute the long-time overaging treatment of a steel strip with compact device by providing a specific number of mandrels which can be independently driven apart an equal angle on the same circumference as the circumference of a revolvable disk in an overaging zone in continuous anenaling.

CONSTITUTION: The steel strip 1 is taken up by the mandrel 3a rotating counterclockwise among 3 pieces of the mandrels 3a to 3c on the above-mentioned disk 2 and the steel strip 1 delivered from the mandrel 3b rotating steel strip 1 clockwise is simultaneously taken up double. On the other hand, either of the steel strips 1 taken up double is delivered by the mandrel 3b but the disk 2 is kept revolved 120° before the coil on the mandrel 3b runs out and the line is once stopped in the state in which the above-mentioned coil is completely run out. The mandrel 3c retreated up to the surface position of the disk 2 is advanced and the mandrel 3b is retreated. The acceleration of the line speed is started and the mandrels 3c, 3b are rotated respectively counterclockwise and clockwise in synchronization therewith. The disk 2 is revolved 120° before the coil on the mandrel 3a is emptied. The above-mentioned cycle is repeated.