The present invention provides a seal ring, for a variable valve timing system, which is resistant to breakage when the seal ring is expanded at a seal ring mounting time even in a case where the seal ring is thick and allowed to have a complicated configuration by performing injection molding and achieve a small oil leak, a low degree of friction, and improved wear resistance. In the variable valve timing system having a camshaft (5) for driving an intake valve of an internal combustion engine and an exhaust valve thereof, an inner rotational body (3) fixed to the camshaft (5), an outer rotational body (4) making a rotational motion relative to the inner rotational body (3) by supply of hydraulic oil at a time of altering opening and closing timings of the intake valve and the exhaust valve, and a shaft body (2) mounted on rotational body (3).