Abstract: An automatic running speed control system for an automotive vehicle in which there are provided a plurality of control switches including a set switch and capable of selectively setting changing drive conditions of a car and a control unit which includes a memory for storing set car speed, a comparator for comparing the actual car speed detected by a speed sensor with the set car speed read from the memory, and an overdrive cut circuit for inhibiting an overdrive condition of an automatic transmission when a fine speed reduction operation is performed by the operation of the set switch or speed reduction switch. With this construction, the automatic constant speed running of a car is performed so as to equalize the actual car speed to the set car speed stored in the memory, while the overdrive condition of the automatic transmission can be inhibited or prevented by the operation of a particular switch such as the set switch.