A hydrostatic continuously variable transmission is provided in which a hydraulic pump and a hydraulic motor are communicated to each other through a low pressure oil passage and a high pressure oil passage and a clutch valve capable of releasing an oil pressure of the high pressure oil passage is connected to the high pressure oil passage. The clutch valve can be automatically operated by making use of an oil pressure of the low pressure oil passage. A clutch valve is applied with an oil pressure of a low pressure oil passage for biasing the clutch valve in a valve opening direction, and an oil pressure governor is connected to a supply oil passage for supplying an oil pressure from an oil supply pump to the low pressure oil passage through a check valve for increasing an oil pressure of the oil passage in accordance with an increase in an input rotational speed of a hydraulic pump.