To prevent a busy shift frequently repeating a shift between an overdrive(OD) shift step and the third-speed step of an automatic transmission during a constant-speed travel.

When a vehicle reaches an uphill from a flat road during a constant-speed travel, the throttle opening is increased, and a transmission control means 9 shifts down the shift step from an OD shift step to the third- speed step and generates and feeds a shift signal to a constant-speed travel control means 10. The constant-speed travel control means 10 has a transmission limiting function for feeding an OD cut request signal to the transmission control means 9 and prohibiting the transmission control means 9 from shifting the shift step, and the constant-speed travel at the third speed is continued. When the vehicle again reaches the flat road or a downhill and the throttle opening is closed to a shift-up recovery opening set value ci or above, the OD cut request signal is released, and the transmission control means 9 shifts up the shift step from the third-speed step to the OD shift step.