PURPOSE: To enable use of old doubling paper strip of having deteriorated tensile strength by specifying positional relation of a deflector roll of a steel strip and a tension roll of doubling paper strip against a winding-up reel.

CONSTITUTION: A deflector roll 2 and a winding-up reel 3 are arranged in nearly lateral direction for a steel strip 1 moving at a speed V. A paper tube 4 on which a doubling paper strip 5 is coiled is provided in the upper position and the strip 5 is supplied by an electrically driven paper feeding speed synchronizing device. The strip 5 is inserted between strips 1 of steel strip coil 7. The angle α made by straight part A of direction of advance of the strip 1 between the roll 2 and coil 7 and straight part B of direction of advance of the strip 5 between the tension roll 6 and the coil 7 is regulated to ≤10°. By this way, the strip 5 is not wound on the outside of the coil 7 but carried along the upper face of the strip 1 and tearing of the strip 5 can be prevented. Better result can be obtained by regulating the angle β made by an imaginary tangent drawn from the center of the paper tube 4 to outer diameter of the coil 7 and extension of the part B ≤±10°.