PURPOSE: To increase and adjust the moment of inertia without varying the frames of principal parts by changing the diametral size or thickness size of a fan support main plate and a side plate with a cooling fan blade remaining specified in size.

CONSTITUTION: The outside dimensions R1, R2 of a main plate 42 of a side plate 45 are selected at a value larger than the outside diameter R of a fan blade 43 with the fan blade 43 remaining specified in size, and main plate 42 and the side plate 45 are protruded in the outer circumferential sections outward from the fan blade 43 while the plate thickness d1, d2 of the main plate 42 and the side plate 45 is made thicker than original designed size required on strength. According to such constitution, the moment of inertia is increased without changing the characteristics of air blasing at all and without augmenting windage loss in a cooling fan 4.