The control method for a constant speed running of a vehicle performs a vehicle speed control for downshifting the gear position of the automatic transmission when the acceleration/deceleration of the vehicle is larger than the determination value during the constant speed running control. In the control method, the automatic transmission includes a plurality of gear positions that can be downshifted during the constant speed running control, and among the plurality of gear positions, the determination value between the gear positions having a relatively large difference in the gear ratio between the gear positions is set to be larger than the determination value between the gear positions having a relatively small difference in the gear ratio between the gear positions.