A search was attempted for the noise source of a multiblade fan by measuring the acoustic intensity using a sound transmittable scroll, and by investigating the state of flow in detail by measuring three-dimensional flow and turbulence at the outlet of an impeller with the aid of a small-sized hot wire probe. As a result, it was clarified that the principal noise sources of a standard multi blade fan are roughly divided into the main flow zone at around 30% from the hub surface of the impeller, and the separation zone near the shroud surface. The former is the noise generated by the turbulence near the blade suction surface in the main flow zone where the blade passage velocity is large. The latter is the noise generated by the turbulence near the blade in the separation zone of the inlet flow.