PURPOSE: To improve the flattening accuracy of the titled device by forming a crowning whose diameter gradually increases at an end of deflector rolls and another crowning whose diameter gradually decreases at respective opposite ends of an intermediate roll and a press roll and moving the intermediate and press rolls in the opposite direction to each other.

CONSTITUTION: A metal strip 5, whose both side end parts are elongated because they are free and have a worse flatness, is fed onto two reflector rolls 31 and is held by an intermediate elongation roll 1 and a press roll 2. Crowning parts whose diameter gradually decreases formed at respective opposite ends are slided to match to both side ends 5a, 5c of the strip 5, which is flattened by depressing the roll 1 to elongate a center part 5b of the strip 5. Variable height parts of the part passed along the crowning part of the roll 1 are also flattened by the crowning part of the deflector rolls 31 to have a better flatness. If the rolls 31 are slided to match to the side end 5c of the strip 5, a more adaptation is obtained.