To provide a foreign matter entry preventing mechanism having a compact structure and not making abnormal noise.

The foreign matter entry preventing mechanism for preventing the entry of dust has a pair of plates (4A, 4B) having an opening (41) at its middle part and being curved in an unstressed state, and a locking means (5) for locking while pressing the middle part of the plates against a given support shaft (1) through the opening of the plates. The pair of plates (4A, 4B) are locked to the support shaft by the locking means with a given stress in the state of the inner surfaces of the curved plates faced each other on substantially the same axis.