To smoothly bite the end part of a rolled material into a roll caliber and to stabilize the rolling formation by forging the tip end part at the biting side of the rolled material into the roll caliber to the width spreading direction with forging dies before biting into the roll caliber.

In the forging dies 4, only the tip end part at the biting side of the rolled material 3 into the roll caliber 1 is spread from the interval C between flanges of the rolled material 3 to the interval D. At the time of spreading the interval by the forging dies 4, the feeding of the rolled material 4 is stopped. The tip end part in the vertical direction of the forging dies 4 is smaller than the interval C between the flanges before spreading so as to enable come-in between both flanges of the rolled material 3, and according to separating in the vertical direction from the tip end part, this tip end part becomes the interval D after spreading the interval. By this method, the biting miss to the roll caliber is not developed and the rolling efficiency is improved.