To provide a balance weight mounting method for allowing easier mounting of a balance weight than conventional one as well as increasing the fitting strength of the mounted balance weight not to removed when a fan is rotated, while balancing the fan if used for high speed rotation.

Both U-shaped faces approximately U-shaped are bent along the curve of a blade 5 in the same angle direction. The face 2b longer than the face 2a has a larger contact area with the blade 5. An insertion port of the balance weight 1 is narrower than the thickness of the blade 5. On both faces, engaging pawls 3 are provided toward the inside of an approximately U-shaped spring for increasing contact points with the blade.