The present invention relates to an apparatus for running a car at constant speed, the apparatus controlling the car to run at a desired speed, wherein by a control circuit (21), both a car-speed deviation and an acceleration are taken as variables, and a pulse-width of pulse signals having a predetermined period is computed from a function for defining pulse-width, and the above-mentioned pulse is given to an actuator (12) for adjusting opening of a throttle valve (13), thereby quickening to settle down to a set car-speed, to enable to stably run at constant speed. Moreover, resetting device (9), (220), second constant-speed-running canceling device (11), (219) which cancel a state of constant-speed-running only by operation of a canceling switch (8), (28), and checking device (10) for checking memory data are provided, thereby improving safety and reliability of control.