A fan device including: a casing unit having an axis and including a casing body that is formed with a receiving space and an air outlet port in spatial communication with said receiving space, a porous plate that is formed with a plurality of through holes and that is disposed in said receiving space to divide said receiving space into an air-flowing room and a noise-reduction room aligned with and spaced apart from each other along the axis, and an air-entering tube that is connected to said casing body and that is in spatial communication with said air-flowing room; and a fan impeller unit mounted in said casing unit, disposed in said air-flowing room of said receiving space and being rotatable about the axis to generate airflow that partially flows from said air-flowing room into said noise-reduction room through said through holes of said porous plate; characterized in that said casing body of said casing unit includes a base and a cover connected to said base to cooperatively define said receiving space, said base cooperating with said porous plate to define said noise-reduction room, said cover cooperating with said porous plate to define said air-flowing room, said porous plate being further formed with an inner tube-communicating through hole spaced apart from said through holes, said air-entering tube peripherally and outwardly extending from and in spatial communication with said inner tube-communicating through hole and extending out of said casing body to the external environment. a casing unit having an axis and including a casing body that is formed with a receiving space and an air outlet port in spatial communication with said receiving space, a porous plate that is formed with a plurality of through holes and that is disposed in said receiving space to divide said receiving space into an air-flowing room and a noise-reduction room aligned with and spaced apart from each other along the axis, and an air-entering tube that is connected to said casing body and that is in spatial communication with said air-flowing room; and a casing body that is formed with a receiving space and an air outlet port in spatial communication with said receiving space, a porous plate that is formed with a plurality of through holes and that is disposed in said receiving space to divide said receiving space into an air-flowing room and a noise-reduction room aligned with and spaced apart from each other along the axis, and an air-entering tube that is connected to said casing body and that is in spatial communication with said air-flowing room; and a fan impeller unit mounted in said casing unit, disposed in said air-flowing room of said receiving space and being rotatable about the axis to generate airflow that partially flows from said air-flowing room into said noise-reduction room through said through holes of said porous plate; characterized in that said casing body of said casing unit includes a base and a cover connected to said base to cooperatively define said receiving space, said base cooperating with said porous plate to define said noise-reduction room, said cover cooperating with said porous plate to define said air-flowing room, said porous plate being further formed with an inner tube-communicating through hole spaced apart from said through holes, said air-entering tube peripherally and outwardly extending from and in spatial communication with said inner tube-communicating through hole and extending out of said casing body to the external environment.