A current terrain acquisition device acquires current terrain information indicating the current terrain to be worked. A controller is configured to decide a virtual design surface that is located below the current terrain and has an inclination angle. The controller is configured to generate a command signal to the work implement of the work vehicle to move the work implement along the virtual design surface. The controller is configured to update the current terrain information with the current terrain acquisition device as the movement proceeds, and change the inclination angle with the updated current terrain information.