PURPOSE: To prevent the oil level of a high pressure side compressor from dropping so as to keep the oil level of each compressor proper by sending most of oil which returns to a first suction pipe back to the high pressure side compressor, and shifting oil to a low pressure side compressor through an oil equalizing pipe.

CONSTITUTION: A second suction pipe 4 is made to branch from the middle of a first suction pipe 3 and the branch end portion 41 of the pipe 4 is made to project into the pipe 3 and a difference is generated between the respective pressure losses of suction passages so that a first compressor 1 is located on high pressure side while a second compressor 2 is located on low pressure side. An elbow-shaped bent 51 is provided to that portion of the first suction pipe 3 which is on the upper side of the branch portion and oil mist is attached to the wall surface of each pipe using centrifugal forces, thereby enhancing the annular flow ratio of oil and reducing the amount of oil flowing into the second suction pipe 4.