PURPOSE: To surely prevent instrusion of dust into a bearing without increasing the man-hour required for assembling and machining work or the number of components, and to prevent the rotational resistance of a rotary shaft from increasing.

CONSTITUTION: A motor housing 12 having an opening 12a at one end thereof is provided, at the other end thereof, with a bearing holding part having an insertion hole and an insertion hole 13a is made oppositely to the insertion hole in a head 13 fixed to one end of the housing. A rotary shaft 14 is held at the opposite ends thereof by first bearing 21 and a second bearing inserted into respective insertion holes. A cooling fan 23 is fitted, at a boss 23a thereof, over the rotary shaft such that the boss 23a abuts against the first bearing having a outer ring 21a being retained by means of a bearing retainer 24 fitted idly to the rotary shaft. An inner ring 24b projects from the inner circumferential edge of the retainer toward the fan and an outer ring 23e is projected from the fan toward the retainer to define a labyrinth along with the boss.