Abstract: The objects of the present invention are to improve the fan efficiency by the decrease of the shaft power of a fan and/or the increase of the pressure rise owing to a fan impeller, which are accomplished by the rational control of the energy based on the tangential velocity of the prerotation flow generating at ‘the region ahead of an impeller’ inside the suction pipe of a fan, and to reduce fan noise by the improvement of air flow at the portion in close vicinity to an impeller inside the suction pipe. The present invention is related to a by-pass unit (5) disposed upstream an impeller to control the energy generated by the air flowing into the impeller (1), comprising a specific region of a suction pipe (2) as a part of the by-pass unit (5), an air chamber (3), through which the air is able to pass, forming a closed spatial region outside the suction pipe (2), and communicating passages (4) for connecting the closed spatial region to the suction pipe (2).