To improve the yield of strip steel plates as a product by reducing the number of scrap when trimming the strip steel plates.

In the same knife setting condition with a former material, the width change operation is carried out (104), and continued till a welding point reaches a cutting position (106). When the welding point reaches the cutting position, the width change operation is carried out, while changing the knife setting condition in response to a following material (108). These operations are continued till the predetermined width change operation is concluded (110). Since the knife setting condition can be changed in a shorter time than the width change operation, the knife setting change operation can be concluded before the width change operation is concluded (till AWC concluding position). It is possible to generate a deterioration of an end surface of the strip steel plate with the width change operation and a deterioration of the end surface with the knife setting condition change operation within the same period.