A constant speed cruise control system of duty ratio control type for approximating an actual car speed to a stored target car speed, by on/off control of a control valve of an actuator to adjust the throttle valve opening degree by an output duty ratio D obtained from a control line having a gradient showing a conversion characteristic of car speed and duty ratio.The system has a controller in whicha set duty ratio DS corresponding to the target car speed is calculated asSD=SD1+(DM-SD1)/nand the output duty ratio D is calculated asD=G.times.V+SDwhereG: gradient of control lineV: car speed deviationDM: integrating element responding quickly to duty ratio changeSD1: integrating element responding slowly to duty ratio changen: coefficientand the controller corrects to integrate the set duty ratio SD in a direction of approximating the output duty ratio D.