An oil separator separating mist oil from processing-target gas includes: a case, a first partition wall and a lower, second partition wall, both partitioning vertically an internal space of the case; a partition dividing a space between the first and second partition walls into an introduction path and a first chamber; a supply hole vertically penetrating the second partition wall from the introduction path to a separation chamber; a communication hole vertically penetrating the second partition wall from the first chamber to the separation chamber; a rotor in the separation chamber; a center-side space in a center part of the rotor, the center-side space communicating with the supply hole through an open, upper part of the center-side space. The rotor separates the mist oil from the processing-target gas flowing from the center-side space to an outer periphery of the rotor, where the separated oil is emitted. The processing-target gas flows into the first chamber through the communication hole.