A vehicle speed control system comprises a section for calculating a command driving force-required to make a sensed vehicle speed equal to a preset command vehicle speed, a section for calculating a command engine output in accordance with the command driving force and the command vehicle speed, a section for calculating a command engine speed corresponding to the command engine output from a predetermined engine torque-versus-speed characteristic relationship in an engine constraint state of minimum throttle setting and fuel cutoff inhibition when the command driving. force is negative, and a section for calculating a command transmission ratio in accordance with the command engine speed. When the driving force command is negative, the control system put the engine in the constraint state and controls a continuously variable transmission in response to the command transmission ratio so as to prevent undesired vehicle speed hunting.