A blade control system includes a first open ratio setting part for setting a first open ratio of a proportional control valve based on a difference angle between a blade angle and a slope angle; a second open ratio setting part for setting a second open ratio of the proportional control valve based on a difference load between a blade load and a target blade load; and a lift controlling part for controlling the proportional control valve in accordance with the second open ratio when the blade load is out of a predetermined load range and for controlling the proportional control valve in accordance with the first open ratio when the blade load is within the predetermined load range.