An oil separator that separates mist oil from processing-target gas containing the mist oil, characterized by: a tubular case having an internal space and an inlet hole which is communicated with the internal space; a first partition wall (22 a) partitioning the internal space into a space with which the inlet hole is communicated and a space located above the space; a second partition wall disposed in the internal space lower than the first partition wall (22 a), the second partition wall vertically partitioning a space of the internal space located lower than the first partition wall (22 a); a partition portion disposed between the first partition wall (22 a) and the second partition wall, the partition portion dividing a space between the first partition wall (22 a) and the second partition wall into an introduction path and a first chamber, the introduction path being communicated with the inlet hole, the first chamber being separated away from the inlet hole; a supply hole disposed inside the introduction path and communicated with above and below the second partition wall; a communication hole disposed inside the first chamber and vertically penetrating through the second partition wall; a rotor being housed in the internal space at a lower position than the second partition wall, the rotor rotating around a vertically-extending rotation axis; and a center-side space disposed in a center part of the rotor, the center-side space being communicated with the supply hole through an upper part of the center-side space which is open, wherein the processing-target gas is introduced from the inlet hole to the introduction path, the processing-target gas flows into the center-side space through the supply hole, the rotor rotates to separate the mist oil from the processing-target gas which is flowing from the center-side space to an outer periphery of the rotor, the separated oil is emitted from the outer periphery of the rotor, the processing-target gas which has discharged from the outer periphery of the rotor flows into the first chamber through the communication hole. a tubular case having an internal space and an inlet hole which is communicated with the internal space; a first partition wall (22 a) partitioning the internal space into a space with which the inlet hole is communicated and a space located above the space; a second partition wall disposed in the internal space lower than the first partition wall (22 a), the second partition wall vertically partitioning a space of the internal space located lower than the first partition wall (22 a); a partition portion disposed between the first partition wall (22 a) and the second partition wall, the partition portion dividing a space between the first partition wall (22 a) and the second partition wall into an introduction path and a first chamber, the introduction path being communicated with the inlet hole, the first chamber being separated away from the inlet hole; the introduction path being communicated with the inlet hole, the first chamber being separated away from the inlet hole; a supply hole disposed inside the introduction path and communicated with above and below the second partition wall; a communication hole disposed inside the first chamber and vertically penetrating through the second partition wall; a rotor being housed in the internal space at a lower position than the second partition wall, the rotor rotating around a vertically-extending rotation axis; and a center-side space disposed in a center part of the rotor, the center-side space being communicated with the supply hole through an upper part of the center-side space which is open, wherein the processing-target gas is introduced from the inlet hole to the introduction path, the processing-target gas flows into the center-side space through the supply hole, the rotor rotates to separate the mist oil from the processing-target gas which is flowing from the center-side space to an outer periphery of the rotor, the separated oil is emitted from the outer periphery of the rotor, the processing-target gas which has discharged from the outer periphery of the rotor flows into the first chamber through the communication hole.