An exhaust gas recirculation system for returning part of exhaust gas of an engine to an intake system has at least one EGR gas introduction port for directing the EGR gas into an intake air passage downstream of a throttle valve. The EGR introduction port opens, into the intake passage, in a tangential direction to produce a circumferential flow along an inside curved surface of the intake air passage around a central back flow region behind the throttle valve to mix the EGR gas efficiently with the fresh intake air and to prevent deposits on the throttle valve.