A silencing device disposed to the outside of a fan 2 that discharges air from the interior of a device 1 to the outside. The silencing device includes a reflection plate 6 provided substantially normal to the rotation axis 2a of the fan 2, and a right-angled parallelepiped shaped duct 4 that guides air from the fan 2 to the outside of the device 1. The reflection plate 6 is disposed at the other end of the duct 4, an air discharge slit 7 is formed in at least one of the four sides of the reflection plate 6, and the distance from the fan 2 to the reflection plate 6 satisfies the relationship L=(.lamda./2).times.n. n is a natural number, and .lamda. is the wavelength of the maximum component from among the wavelength components that constitutes the sound generated by the fan 2.