To prevent the breaking of a sheet and the damage of a press plate by securing weld strength and preventing the increase of roll load at endless rolling.

In a method by which sheet bars before finish rolling are joined and finish rolling is continuously performed, the thickness data of the sheet bar which are imparted to each sheet bar to be joined in the stage of an instruction set are compared in a group of joining sheet bars and all the thickness data of the sheet bars in the group are changed and unified into the thickness of the thinnest sheet bar.