In a shift control system of an automatic transmission, wherein the shift control system includes at least a first and a second transmissions capable of automatically switching speeds separately of one another, and the first and the second transmissions are shifted simultaneously or alternately, to thereby achieve multi-speed shifts, there is provided means for starting and completing changes in rpm for the shift of rotary members of the second transmission during the operation of changes in rpm for the shift of rotary members of the first transmission, particularly when the first transmission is low gear shifted and the second transmission is shifted simultaneously, whereby the automatic transmission as a whole is down shifted, so that the shift characteristics can be maintained satisfactorily.