PURPOSE: To optimally relax a sheet as a whole by releasing tensile force by forming at least a single free loop in the drawing, wetting and relaxation treatment-applied sheet before being hardened in a low temperature zone.

CONSTITUTION: A polyvinyl butyral sheet 6 moves by being drawn between an upper roller 4 and a lower roller 5 in wetting/relaxation chambers 2, 3 of a high temperature zone 1, and is hardened by passing through a low temperature zone 7 to be wound by a winder 12. Before entering this low temperature zone 7, the sheet 6 is heated to a prescribed temperature by a heating means 21, and a free loop 17 is formed up to a position of a detector 18 by dead weight between free loop forming rollers 15, 16 so as to be sufficiently relaxed by being released from tensile force between the rollers. Thus, when the polyvinyl butyral sheet is used for laminated glass, contraction at collecting time can be ignored since the contraction is promoted before winding the sheet.