In a rotary hydraulic actuator wherein a hermetic casing has the inside divided into a first hydraulic chamber and a second hydraulic chamber by a vane slidably rotatable on an inner curved wall of the casing, the vane is rotatable by an oil pressure supplied to the respective chambers, and the movement of the vane is taken out from an output shaft, the inner wall of the casing which is in sliding contact with the vane is formed with a polytetrafluoroethylene sliding member.