A current terrain acquisition device acquires current terrain information. The current terrain information indicates the current terrain to be worked, and includes the height of the current terrain at a plurality of points. A controller is configured to decide the smoothed height for each of the plurality of points. The controller is configured to decide a smoothed current terrain that includes the smoothed height of the plurality of points. The controller is configured to decide a virtual design surface on the basis of the smoothed 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 virtual design surface.