To improve a flywheel attaching and detaching control method and device of a flying shear which has a flywheel set at the input side of the finish rolling mill of a hot rolling line to manufacture a hot rolled steel strip.

This flywheel attaching and detaching device of a flying shear is composed of an warping amount detecting device 10 to detect the upper warping amount of a work beforehand, a size and specification setter 11 to set the size and the specification of the work, and a flywheel connecting and removing discriminating device 12 which calculates the necessary cutting force from the size and the specification and the upper warping amount of the work, and discriminates and instructs whether the flywheel is necessary or not from the necessary cutting force.