Abstract: A method of changing the width of a continuous metal casting of rectangular cross section without interrupting the casting, wherein a length of the continuous casting is cast through a mold defining a cavity of rectangular cross section having a longitudinal axis, the mold having two small side walls determining therebetween the width of the continuous casting, at least one of the side walls being displaceable with respect to the longitudinal axis to change the width, which comprises the improvement of so controlling the displacement of the one side wall in response to the length of the continuous casting cast since the beginning of the width changing operation that a transition zone of changing width of the continuous casting has a predetermined length regardless of any change in the casting speed of the continuous casting during said operation.