PURPOSE: To remove, with certainty, the occurrence of waviness in a web at the time of heating a wide flange shape.

CONSTITUTION: Rollers 2, cylinders 3, or members 4 composed of a material higher in coefficient of thermal expansion than a wide flange shape 1 are disposed between two flanges 1a, 1a of the wide flange shape 1 so that the distance between two flanges 1a, 1a mentioned above is extended. At the time of heating, heating is applied to the above wide flange shape while applying tensile stress to a web 1b in a web-height direction.