second intermediate shaft 4 to a final reduction gear 5 the rest of the rolling mill is the same as it was before the modification as a result thin materials such as those shown at nos 1 and 2 in table 2 which are processed at an extremely low efficiency by conventional rolling are efficiently rolled by the above arrangement thus all the materials shown in table 2 can be processed without having to increase the capacity of a conventional direct current motor having a continuous rated output which is only 55 of the continuous rated output required when materials of differing thicknesses are processed at appropriate production efficiencies when employing a single rolling mill plant whose rolling speed ratio is 2 5 as the conventional level the results obtained by this processing are the same as those obtained by a rolling mill plant using direct current motors of a desired continuous rated output level recent improvements in the performance of semi conductors and computers have made the process of converting the frequency of a power source easier the controllability of alternating current motors are thereby so greatly improved that they can be used as motors for driving a rolling mill plant regarding speed control of alternating current motors there are pole changing control and frequency changing control the pole changing control is not used except for discontinuous speed of alternating current motor and so such frequency changing control is indispensable for the continuous mill plant for rolling steel plates with minute.