PURPOSE: To improve the machinability and machining accuracy of the title diffuser by connecting a second side plate to an integrated body consisting of a first side plate and a vane so as to enable the rotary shaft of a rotary cutter for machining a vane to be set in a state perpendicular to the first side plate for carrying out its machining work.

CONSTITUTION: A vane 4 used in a centrifugal compressor is formed so as to become thicker as it approaches a first and second side plates 5, 6. An air passage 11 is formed to have a non-circular cross section or a drum 1 shape cross section and also formed in retreating from end parts 12a, 12b in the axial direction at the front edge of a vane 4. While, the second side plate 6 is connected to the first side plate 5 and the vane 4.