To provide a fan retention structure capable of greatly reducing fan vibration and making low-frequency noise caused by the fan extremely low.

This structure is provided with flanges 2a, 2a on a suction side and a delivery side tubular shape frames 2d of an axial flow fan 2, respectively, and with a vibration isolation sponge 10 surrounding the outer circumference surface of the frames and installed between the flanges 2a, 2a. Depressed mounting frames 12, 12 are provided on the upper surface and the lower surface of the vibration isolation sponge 10, the upper surface and the lower surface of the vibration isolation sponge 10 mounted to the axial flow fan 2 are nipped between top and bottom inner surfaces of a casing via the mounting frame 12, and the axial flow fan 2 is retained in the casing.