PURPOSE: To provide a suction/exhaust cylinder adapted to keep a pressure between an inlet and an outlet well-balanced relative to an external wind, while recirculating a minimum quantity of exhaust gas.

CONSTITUTION: A suction/exhaust cylinder B is constituted in such a manner that a double cylinder portion constituted by a first outer cylinder 7 and an inner cylinder 6 has its outer passageway formed as a suction air passage 13 which communicates with an atmosphere via an inlet port 12 defined in a inlet wall 11, and its inner passageway formed as an exhaust air passage 15 which communicates with an atmosphere via an outlet port 14 defined in an outer periphery adjacent to the top end of second outer cylinder. Furthermore, a wind pressure control board 17 having a dimension larger than the outer diameter of second outer cylinder 8 is provided to the first outer cylinder 7. The wind pressure control board 17 can increase a static pressure, but nevertheless keep the difference N of pressure between the inlet port 12 and the outlet port 14 at constant even when the suction/exhaust cylinder B has its projecting length from the attaching wall surface thereof varied.