A terrain information acquisition device acquires terrain information about a work site. A controller is configured to decide an inclination angle of the current terrain along a travel direction of the work vehicle from the terrain information. The controller is configured to decide a virtual design surface that is inclined at an inclination angle smaller than the inclination angle of the current terrain. The controller is configured to generate a command signal to a work implement of the work vehicle to move the work implement along the inclined virtual design surface.