The present invention provides a load controller for use in a hydrostatic transmission for a work vehicle that is free from control failure caused by deterioration of the constituent components, and that can be miniaturized and simplified in structure compared to conventional load controllers. In the work vehicle, power supplied from an engine (E) is branched to drive an implement system and a traveling system. A hydrostatic transmission (1) is provided in the traveling system. The hydrostatic transmission (1) connects, in an oil-hydraulic closed circuit, an oil-hydraulic pump (2) driven by the engine (E) and an oil-hydraulic motor (4) driven by the oil-hydraulic pump (2). The load controller is provided with a bypass oil line for bypassing an oil line that has high pressure during forward movement (3a) in the oil-hydraulic closed circuit to reach an oil line that has low pressure during forward movement (3b) or an oil tank (T); a first opening/closing valve for opening or closing the bypass oil line (8); and an opening/closing valve (9) controller that detects a load applied to the engine (E) while traveling during working and opens the first opening/closing valve (9) when the load exceeds a predetermined level.