The purpose of the present invention is to enhance the efficiency in removing oil contained in target gas in an oil separator that separates oil mist contained in target gas from the gas. The oil separator 2 according to the present invention includes a cylindrical housing 11 that accommodates a rotor and that is provided with a gas discharge portion 24 (first exhaust portion), the oil separator being configured to introduce target gas from a rotation center side of the rotor to condense oil mist and emit condensed oil from an outer peripheral edge of the rotor and further being configured to discharge from the housing 11 through the gas discharge portion 24 the blow-by gas (target gas) after having the oil mist separated. The oil separator 2 is characterized to include a dome portion 61 (sectioning member) that covers from an outside of the housing 11 the gas discharge portion 24 and sections a closed space around the gas discharge portion 24 and an outlet pipe 62 (second exhaust portion) that is provided to the dome portion 61 and that discharges the blow-by gas from the closed space that is sectioned by the dome portion 61.