PURPOSE: To prevent liquid from being compressed by oil in normal operation by boring at the utmost low position of a vane casing an oil discharge hole, which is closed by a flapper valve under a discharge pressure in said operation to achieve a compression stroke and opened to discharge the oil during recess.

CONSTITUTION: By forming an oil discharge hole 27 and a flapper valve 50 at the lowest position of a vane casing 39, reflux oil 46' deposited in a cylinder 29 is removed from the cylinder 29 during recess of a compressor so that liquid may be prevented from being compressed to yield a high pressure at the start of revolution. Since the flapper valve 50 is closed by a discharge pressure P2 in normal revolution, a normal compression stroke may be executed. At the same time, the oil 46" flowing out of the cylinder 29 is mixed with discharge gas under the influence of its pressure or flowing energy, sent to an oil separator, and then stored in the back space of a shell 29 to be used again as lubricating oil.