An injection timing control system for a fuel-injection pump for an engine has a phase angle regulator for varying the rotational phase angle of a drive shaft of the fuel-injection pump in which a cylindrical extension of one of an input shaft adapted to be driven by the engine and an output shaft for driving the drive shaft of the pump surrounds an end portion of the other shaft in spaced apart relationship. A male spline is formed on the end portion over some axial length thereof while a female spline is formed on the extension over some axial length thereof, at least one of the splines being a helical spline. A slider is located in an annular space between the splines and formed with splines meshing therewith respectively. An annular cylinder is formed between the substantial parts of the rest axial length portions of the extension and end portion, and a piston is reciprocally received in the cylinder, the slider and piston being formed as a unit. The system also has a hydraulic pump for supplying pressurized fluid to the cylinder, the hydraulic pump being mounted on a casing enclosing the extension and rotatably supporting the one shaft and extension and being operatively connected to the extension through gear means to be driven by rotation of the extension.