PURPOSE: To prevent foreign substances such as dust, dirt, moisture content or water content, etc., from penetrating into the inner part of a fan by installing each groove, going along a direction of rotation, on the inner circumferential surface of a support base substance of a fin and the outer circumferential surface of a tubular projection of a fan cover.

CONSTITUTION: Each of grooves 81-83 going along a direction of rotation is installed in at least one side of the inner circumferential surface of a support base substance 101 supporting a fin 102 and the outer circumferential surface of a tubular projection 203 provided in a fan cover 2. With this constitution, a pressure gradient serving as prerequisite for penetrating fine dust and dirt into the inner part of a fan 1 is interrupted by these grooves 81-83. Accordingly, such a possibility that foreign substances such as dust, dirt moisture content or water content, etc., might be penetrated into the fan inside can be checked.