To eliminate the noise problem by forming the inner structure of a shell such that refrigerant gas flows as a laminar flow in streamline along the inner wall of the shell, thereby preventing generation of swirling sound.

A tubular shell having a diameter larger than that of the piping is provided with an inlet on one end side and an outlet on the other end side thereof while being located on the opposite sides of the central axis of the shell. The opposite side walls of the shell are symmetric to the central axis of the shell and incline at a specified angle. The inlet made in one side wall and the outlet made in the other side wall are located at different levels on the long axis of inclining face of each side wall on the opposite sides of the central axis of the shell. Fits are provided by drawing from the shell between the side walls and at the roots of the inlet and outlet on the side wall such that the refrigerant flows in streamline.