PURPOSE: To reduce the reexpanded gas volume and enlarge the delivery port area of a machine by making the involute angle in a cross section perpendicular to scroll axis the nearer the end plate the smaller in the fixed side and the nearer the end plate the larger in the rotating side respectively.

CONSTITUTION: Involute beginning angles E1WE3, B1WB3 of the central spiral body 1 of a fixed scroll are made smaller the more the body 1 nears the fixed scroll end plate 4. Similarly, involute beginning angles of the central spiral body of a rotating scroll are made larger the more the body nears the rotating scroll end plate the larger. Thereby a delivery port 5 in the fixed scroll side can be enlarged. And the tip portion of the spiral body of the rotating scroll is thinned. Thus the gap producing a small chamber between both spiral bodies can be eliminated to improve performance.