A system and method for automatically controlling a cruising speed of a vehicle which can maintain a vehicle speed at a desired cruising speed is disclosed in which a plurality of initialization signal values are set after an accelerator switch installed in a command switch is initially operated and thereafter is released and again operated to command a desired acceleration of the vehicle irrespective of the type of vehicle using the system, i.e., according to the vehicle speed. Second and third initialization signal values have predetermined functional relationships to a first initialization signal value which is set when the accelerator switch is initially operated. These signal values are, e.g., time widths for which a vacuum supply valve of a throttle valve actuator is opened so that an engine throttle valve is opened to increase the vehicle speed.