PURPOSE: To increase the natural frequency of the series of an oil separator and improve the oil separating performance thereof by a method wherein a partitioning plate, equipped with an air path, is provided between the compressor block and the oil separator element.

CONSTITUTION: The shell 1 of the oil separator is divided into two chambers 2, 3 by the partitioning plate 4 equipped with the air path 5. When the compressor in the compressor block 7 is driven, the air, having an atmospheric pressure, is compressed and is discharged into the chamber 2 through a discharging pipe 12. The compressed air, discharged into the chamber 2, repeats collisions in a cavity between the compressor block 7 and the partitioning plate 4 and the oil is separated thereby finishing a primary separation. The compressed air, finishing the primary separation thereof, enters into the chambers 3 after passing through the air path 5, the secondary separation of the compressed air and the oil is effected by the oil separator element 8 and, thereafter, is discharged from the discharging port 14.