The invention relates to a screw rod compressor, which is equipped with: a double circular cylinder integrated with the compressor cast for rotary separating oil-gas and a passage in a rotary passage formed by the double circular cylinder for conducting compressed air. The screw rod compressor has characteristic in that: the double circular cylinder inner bottom has a thin front end shape, and front end of the inner bottom is flat. Thus, lubrication oil flowing to the outer face side of the inner cylinder of the double circular cylinder becomes oil droplets and during centrifugal separation without adhering to the bottom end of the cylinder and the droplets are allowed to fall so that the oil is prevented from rising along the wall. Accordingly, separation performance can be improved.