An acoustic damper is fixed to a vibration source to be provided along an outer surface of the vibration source, forms a passage that takes in air vibration generated by the vibration source, and is provided with an acoustic portion having a partition member closing the downstream side of the passage with respect to the propagation of the air vibration to serve as resistance against the air vibration, in which the acoustic portion allows air vibration to propagate in a reverse direction, and has partition members arranged so as to serve as resistance against the air vibration in respective directions. This allows to prevent from generating stress due to pressure fluctuation caused when reducing vibration, and to achieve a relatively compact size.