A current terrain acquisition device acquires current terrain information indicating a current terrain to be worked. A controller is configured to generate an inclined virtual surface extending from a current position of the work vehicle. The controller is configured to decide an estimated held soil volume of a work implement of the work vehicle when the work implement is moved along the virtual surface in the current terrain. The controller is configured to decide whether the estimated held soil volume has reached a predetermined held soil volume threshold. The controller is configured to set the virtual surface extending from the current position of the work vehicle when the estimated held soil volume has reached the held soil volume threshold as a first virtual design surface, and generate a command signal to the work implement to move the work implement along the virtual design surface.