A structure of a ventilation fan provided with a plurality of blades has a performance curve characterized by a gentle slope throughout the wide operating range instead of an S-hysteresis curve, in order to overcome a fluctuation phenomena of the operating flow rate. The blade tip cross-section lies on a first imaginary cylindrical surface and an angle of attack and pitch angle of the blade tip are formed as respective trajections by intersecting the first cylindrical surface by a second cylindrical surface whose center axis has been rotated about two axes.