In a hydrostatic continuously variable transmission, a hydraulic pump and a hydraulic motor are connected through a hydraulic closed circuit and the capacity of the hydraulic motor is varied to vary the speed continuously. The transmission has a valve spool in a spool hole made in a transmission output shaft which rotatably holds the hydraulic pump and hydraulic motor; and the transmission output shaft incorporates outer branch oil paths which are connected with an outer passage and open to the spool hole, and an inner branch oil path which is connected with an inner passage and open to the spool hole. A right groove on the outer periphery of the valve spool enables the valve spool to move between a clutch release position in which the outer branch oil paths and the inner branch oil path are connected and a clutch engagement position in which a central land of the valve spool at least covers a high pressure clutch port; and the right groove is connected to the outside through a discharge oil path with an orifice.