A fan filter unit includes a housing having a side wall and a top wall with a bell mouth serving as an air inlet and an open bottom to which a filtering net is attached to serve as an air outlet. A centrifugal fan is arranged in the housing to drive the air from inlet. The fan assembly has an output port for driving an air flow toward the air outlet. A guide plate is arranged in the housing for guiding the air flow along the path from the fan assembly to the air outlet. A sound-absorbing structure including a plurality of wedge-shaped elements made of a sound-absorbing material is attached to the side wall. The wedge-shaped elements are arranged in a line for effectively reducing noise level caused by the operation of the fan assembly. A second line of wedge-shaped elements is optionally attached to the side wall with the wedge-shaped elements of the two lines alternating each other.