PURPOSE: To perform a very quick rise-up of pressure at selection and improve a control response speed, by holding the displacement space of an oil hydraulic actuator to a condition under confined pressure in the neutral position of an oil hydraulic selector valve.

CONSTITUTION: An oil hydraulic actuator 2 controls a relatively rotational phase angle between an input shaft 1a, driven by an engine, and an output shaft 1b, driving a fuel injection pump 3. While displacement chambers 2a, 2b are provided in the oil hydraulic actuator 2. Then back pressure is produced in a flow line, from an oil hydraulic pump 5 to a port 4c, in a neutral switching position 4B of an oil hydrualic selector valve 4, further the displacement chamber 2b is held to a condition under confined pressure, thus working oil in these flow lines is held under a condition approximately to non-compressibility. In this way, at selection from the switching position 4B to 4D, a rise-up of pressure in the displacement chamber 2b can be performed very quickly.