PURPOSE: To perfectly detect abnormality even in a low speed region, by detecting the abnormality to be decided through an on-off condition of light provided incident to as to cross a cooling fan.

CONSTITUTION: A beam of light, emitted from a light emitting element 11 provided in a cooling fan case 1, goes straight to reach a light receiving element 12. A cooling fan driving electric motor 3 is placed in an abnormal condition, and when a cooling fan 2 is restricted, a control box 14 detects the light, which is placed in a shielded condition or a permeating condition to reach the light receiving element 12, to be decided. Consequently, a protective device, enabling abnormality to be perfectly detected even in a low speed region, surely protects the cooling fan.