Guiding speed is increased while an unmanned vehicle is prevented from straying from a travel passage width, to improve the work efficiency. Target speed of the unmanned vehicle increases as a distance between a current position of the unmanned vehicle and a guidable borderline increases, and the target speed of the unmanned vehicle decreases as the distance between the current position of the unmanned vehicle and a guidable borderline decreases. The unmanned vehicle travels, along a travel path having adjacent inbound/outbound lanes, in a direction opposite to a direction of a vehicle on an oncoming lane. If it is determined that the vehicle traveling along the oncoming lane is approaching the unmanned vehicle, then the target speed of the unmanned vehicle is reduced, whereby the unmanned vehicle is caused to travel at a low guiding speed.