A silencing device and an air supply system according to an embodiment of the present invention include a resonance silencer provided at a position connected to a space in which the sound source is located within the air supply system in order to silence the noise generated from the sound source in the housing, in which in a case where a resonance wavelength of the resonance silencer alone is denoted by λ, a distance by which the resonance silencer is separated from the sound source is less than λ/2. A fundamental resonance frequency of the resonance silencer is equal to or less than an acoustic upper limit frequency determined according to a size of the housing.