An oil separator includes a separation container. The separation container has a side surface portion to which an inlet pipe is attached. The separation container has an upper surface portion to which an outlet pipe is attached. An oil reservoir is provided in a lower portion of the separation container. The separation container has a lower surface portion to which an oil return pipe is attached. The separation container has an inner wall surface provided with a liquid passage section. The liquid passage section is provided with a groove. The groove is disposed to extend in a direction of gravity toward the oil reservoir. The groove is formed to be gradually increased in depth from an upper portion of the groove toward a lower portion thereof.