Provided is an oil separator built-in compressor, which prevents a drop in the separating ability of an oil separator while reducing a pressure loss in a communication hole formed between a discharge chamber and an oil separation chamber and which also considers the working feasibility of a casing. The oil separator built-in compressor comprises: a separation chamber arranged adjacent to the discharge chamber and formed entirely thereof into a space in order to separate an oil-containing gas being introduced, centrifugally into gas and oil so that the separated oil flows downward whereas the separated gas is released upward; and the communication hole formed between the discharge chamber and the separation chamber in order to introduce the oil-containing gas from the discharge chamber into the separation chamber. The oil separator built-in compressor is characterized in that the oil separator is formed of the joint structure of two members for constituting the compressor, and in that a portion of the inner wall face of the communication hole is formed at a position farther from the joining face of the joint structure than the inner wall face of the separation chamber.