A compressor includes a compressing mechanism for compressing refrigerant gas and an oil separator for separating the oil from the gas. The separated oil is used to lubricate the compressor. The compressor has a discharge passage to permit refrigerant gas to flow out of the compressor, a recess located in the discharge passage, a plug press fitted in the recess and a supply passage for returning the separated oil to the compressor. The plug and the recess define a separation chamber having a circular cross-section and an annular chamber. The separation chamber is connected with the annular chamber by an outlet passage formed in the plug. The refrigerant gas swirls along the wall of the separation chamber, which separates the oil from the gas. Since the plug is press fitted in the recess, installation of the plug is facilitated. This structure also prevents the plug from loosening.