A system and method for automatically controlling a vehicle speed to a desired cruise speed are disclosed in which in addition to a control unit and a throttle actuator through which an opening angle of an engine throttle valve is adjusted so that the vehicle speed detected by a vehicle speed sensor coincides with a target cruise speed set by a vehicle driver through a set switch, a vehicle speed determining circuit is provided for determining whether the vehicle speed detected by the vehicle speed sensor exceeds a lowest speed limit above which the cruse control by the control unit and throttle actuator becomes effective. When the vehicle speed is below the lowest speed limit, the power supply to the throttle actuator is interrupted so that the cruise control by means of the throttle actuator is cancelled.