PURPOSE: To remove a wave existing on a sheet web by holding the web at a substantially lateral extending temperature, guiding it without lateral movement until the wave is eliminated to become completely flat, and then cooling it by a cooling roller.

CONSTITUTION: A sheet web 2 is guided at a sheet web winding angle if 150° at a maximum along outer peripheries of discs 8, 8', and laterally extended at a ratio having a lateral shrinkage of 8% at a maximum of the wound sheet. Then, after the web 2 is output from a lateral extending unit, a wave existing on the web is removed at a passage thereafter by first holding it at substantially lateral extending temperature, guiding it without laterally moving until the wave is eliminated to become completely flat, and then cooling it by cooling rollers 12, 13. Then, the wound sheet is guided by a tension to have a longitudinal shrinkage of 10% at a maximum.