a rolling load signal is detected during a few rotations of a top and a bottom backup roll at different detection time points at which the top and bottom backup rolls are out of phase from each other and then are analyzed by fourier analysis so as to detect the amplitudes and phases of the eccentricity of the top and bottom backup rolls separately , whereby the roll gap is controlled and the separately detected eccentricity of the top and bottom backup rolls is thus obtained . even when there is a difference in roll eccentricity frequency between the top and bottom backup rolls and even when external disturbances exist due to the aging of the roll eccentricity and to the estimated errors of the mill constant m and the plasticity coefficient q of a piece of metal to be rolled , the roll eccentricity can be suitably adjusted so that the roll eccentricity can be detected with a high degree of accuracy and then eliminated .