PURPOSE: To provide the mandrel winding method where the pressing time for a mandrel by a wrapper roll is made shortest without damaging a coil in winding a strip round the coil by using the two-stage expansion and reduction diameter mandrel.

CONSTITUTION: The minimum number of windings where tensile stress τ2 generated on coil inside winding for the number of windings of the coil from starting of winding of the coil 6 for every strip becomes less than yield stress of the strip in the winding temperature is obtained by calculation and when this number of windings is attained, the second stage mandrel diameter expansion is executed and the wrapper roll is released from the mandrel. τ1 and a caption number 8 show mandrel diameter expansion stress and a coil cross section, respectively.