A filtering device is adapted to be mounted in a fan having a motor cover, a rotary shaft, an impeller combined at the rotary shaft, and a rear protective grill mounting on a mounting base provided on a front end of the motor cover. The filtering device includes an inner ring plate, an outer ring plate, and a filter element. The filter element is sandwiched between the inner ring plate and the outer ring plate, and the inner ring plate and the outer ring plate are coupled between the rear protective grill and the rear protective grill lock ring. The airflow sent out by the rotation of the impeller of the fan will pass through the filter element of the filtering device, achieving the effect of purifying air.