An oil separator configured to separate an oil mist included in a gas which is a target of treatment comprising: a plurality of separation discs which are provided rotatable together with a spindle and are layered in an axis direction of the spindle; a nozzle positioned below the separation discs and configured to rotate the spindle about an axis line by injection of an oil; a lower case which is provided with a gas inflow part into which the gas being the target of treatment flows and an oil discharge part into which an oil after separation is discharged; an upper case which is attached from above the lower case and forms together with the lower case a housing chamber in which the spindle, the separation discs and the nozzle are housed; and a sectioning member which sections the housing chamber into a primary separation chamber and which is configured to allow an oil injected from the nozzle to flow down as well as to primarily separate the oil mist included in the gas being the target of treatment which has flown in from the gas inflow part and into a secondary separation chamber in which the separation discs are arranged and which secondarily separates the oil mist included in the gas being the target of treatment from which the oil mist has been primarily separated, and at the same time said sectioning member forms between the nozzle and the separation discs a communication opening which guides into the secondary separation chamber the gas being the target of treatment in the primary separation chamber, characterised in that said nozzle protrudes from a circumferential surface of the spindle, and in that a path is formed through which the gas being the target of treatment in the primary separation chamber is introduced into the secondary separation chamber through a position on inner side of a tip of the nozzle, said inner side being the side closer to the axis direction, at a height of the nozzle. a plurality of separation discs which are provided rotatable together with a spindle and are layered in an axis direction of the spindle; a nozzle positioned below the separation discs and configured to rotate the spindle about an axis line by injection of an oil; a lower case which is provided with a gas inflow part into which the gas being the target of treatment flows and an oil discharge part into which an oil after separation is discharged; an upper case which is attached from above the lower case and forms together with the lower case a housing chamber in which the spindle, the separation discs and the nozzle are housed; and a sectioning member which sections the housing chamber into a primary separation chamber and which is configured to allow an oil injected from the nozzle to flow down as well as to primarily separate the oil mist included in the gas being the target of treatment which has flown in from the gas inflow part and into a secondary separation chamber in which the separation discs are arranged and which secondarily separates the oil mist included in the gas being the target of treatment from which the oil mist has been primarily separated, and at the same time said sectioning member forms between the nozzle and the separation discs a communication opening which guides into the secondary separation chamber the gas being the target of treatment in the primary separation chamber, characterised in that said nozzle protrudes from a circumferential surface of the spindle, and in that a path is formed through which the gas being the target of treatment in the primary separation chamber is introduced into the secondary separation chamber through a position on inner side of a tip of the nozzle, said inner side being the side closer to the axis direction, at a height of the nozzle.