The purpose of the present invention is to increase the elimination efficiency for oil included in gas to be processed in an oil separator for separating the mist-state oil, which is included in the gas being processed, from the gas. This oil separator (2) has a cylindrically shaped housing (11) in which a rotor is accommodated and a gas discharge unit (24) (first exhaust unit) is provided. By introducing gas to be processed from the side of the center of rotation of the rotor, the mist-state oil is aggregated and released from the outer peripheral edge of the rotor. The blow-by gas (gas to be processed) after the separation of the mist-state oil is discharged from the housing (11) through the gas discharge unit (24). This oil separator (2) is characterized by having: a dome part (61) (defining member) that covers the gas discharge unit (24) from the outside of the housing (11) and defines the closed space at the periphery of the gas discharge unit (24); and an outlet pipe (62) (second exhaust unit) provided in the dome part (61) for discharging the blow-by gas from the closed space defined by the dome part (61).