A valve timing control system includes a hydraulic-pressure supply/discharge device for selectively supplying and discharging working fluid from advance and retard chambers, an annular recess formed in a sidewall member of a housing to face a housing main body and communicating with one supply/discharge passage of the hydraulic-pressure supply/discharge device, and a radial groove radially extending from the annular recess and communicating with the advance chamber or the retard chamber.