PURPOSE: To fix and release a door body easily and speedily and, at the same time, to ensure the fixing of the door body by driving a driving means to bring a taper of a lock member into contact with a taper of the door body or to release the contact, and fixing or releasing the door body.

CONSTITUTION: A taper 40 is formed on the open end of a holder 16 to pivot the end thereof on one post 32 of the base through a bearing member 36 so that it is capable of turning. A positioning member 42, a lock member 46 placed near thereto and forming a taper 46a corresponding to the taper 40 and a hydraulic cylinder 44 as a driving means to advance and retreat the taper 46a against the taper 40 so that it is capable of interlocking and separating are provided to the other post 34, and a fixing device of the holder 16 is formed together with the taper 40. The holder 16 is speedily fixed and released by simple operation only to forward, reverse and energize the hydraulic cylinder 44, and the exchange of a guide bar facing, the inspection of a corrected roll, etc., are made within a short period of time.