PURPOSE: To improve the operability of an acceleration pedal for a temporary acceleration in constant speed traveling, and improve the safety when a drive means for the acceleration pedal fails.

CONSTITUTION: A constant speed traveling device for a vehicle comprises an acceleration pedal 4 operated by a driver, a shaft displaced by a stepping quantity to the acceleration pedal in a rotating direction, a potentiometer to output a stepping quantity signal corresponding to a rotation position of this shaft 17, and a stopper 16 to decide a release position of the acceleration pedal, where the potentiometer and the stopper 16 are fixed. The device also comprises a lever 12 rotatable to the shaft 17, a negative pressure actuator 35 to move the position of the potentiometer and the release position of the acceleration pedal 4 by changing the rotation position of the lever 12 at the time of auto- drive in directions to increase the depression quantity to the acceleration pedal 4, etc.