PURPOSE: To obtain uniform structural yarn of plural layers having a stable covering ratio of yarn of multilayer structure without expensively spending a devise cost and an energy cost.

CONSTITUTION: Sliver S1 as a core is fed to the outside of a draft zone when watched from a coiler head 5. Sliver S2 at a winding side is supplied to the inside, the sliver S1 as the core is treated in such a way an angle θ between the extended line of the sliver S1 in the draft axis direction and a line connecting a nip point Y of a front roller 4 to a coiler head 5 is projected to the horizontal plane to give a 0-60° range. The constituent yarn of the sliver S1 as the core used has higher draft resistance than the constituent yarn of the sliver S2 at the winding side or the sliver S1 as the core is spun at a lower spinning speed than the sliver 2 as the winding side to stabilize the covering ratio of yarn of multilayer structure.