To provide an oil separator with a built-in compressor capable of stably feeding a lubricant to a compression driving part and enhancing separating performance of the lubricant contained in a compressed refrigerant gas.

An oil separating chamber 21 combined with the rear of a compressor housing 1 and formed by a separator cover 2 has a U-shaped channel formed by partitioning its upper end part by a guide wall 22. An inflow port 13 for a mixed refrigerant gas containing a lubricant to flow in and an outflow port 14 to send the refrigerant gas to the condenser side are formed on both sides of the guide wall respectively. An oil gathering part 17 to store the lubricant is recessed at the lower part of the oil separating chamber, and the oil gathering part is in communication with the inlet of an oil collecting passage 31 formed in a gasket 3 interposed between the compressor housing and the separator cover, and the outlet of the oil collecting passage is in communication with an oil feeding passage 16 formed in the upper end part on the rear wall surface of the compressor housing so as to be in communication with a crank chamber 18. An oil separating plate 4 and/or an oil separating net 5 can be also installed.