An automotive constant speed cruise control system for a vehicle comprises a controller for automatic speed change for changing over automatically speed stage in a transmission by a relation between a throttle valve opening degree and a car speed, a controller for constant speed traveling having a constant speed traveling control function for cruising a car at a target car speed by controlling based on a duty ratio the throttle valve opening degree according to a traveling car speed and a transmission limiting function for sending out a signal for prohibiting selection of maximum speed stage in the transmission to the controller for automatic speed change for a specified time when the car speed lowers, and the transmission limiting function capable of being started from outside by inputting a speed change signal representing that the transmission is shifted down from a maximum speed to the controller for constant speed traveling.