An acoustic damper (1A, 1B, 1C) fixed to a vibration source along an outer surface of the vibration source which, in operation, generates air vibration, the acoustic damper (1A, 1B, 1C) comprising: a housing surrounding the outside of the outer surface of the vibration source so as to form a space on the outside of the outer surface, wherein a plurality of through holes are formed in the circumferential direction of the vibration source at the portion where the housing is provided that allow air vibration to pass to the outside of the vibration source and into the space; characterised by: a compartment wall arranged in the housing so as to partition the space formed by the housing in an axial direction along an outer wall portion of the housing to form two adjacent passages that takes in air vibration generated by the vibration source; a partition wall arranged so as to respectively further partition the adjacent passages into an inner space adjacent the outer surface of the vibration source and an outer space remote from the outer surface of the vibration source, wherein each outer space is respectively provided with an inlet communicating with the inner space, with a first partition member forming an end portion of the passage, and with a second partition member serving as resistance against the air vibration, and closing a downstream side of the passage with respect to a propagation direction of the air vibration in an acoustic portion configured as the space from the inlet at the end portion along the direction of the passage to the second partition member, wherein the inlet and the first and second partition members (5a,5b) are arranged such that the propagation directions of the air vibration in the adjacent passages are reversed to each other. a housing surrounding the outside of the outer surface of the vibration source so as to form a space on the outside of the outer surface, wherein a plurality of through holes are formed in the circumferential direction of the vibration source at the portion where the housing is provided that allow air vibration to pass to the outside of the vibration source and into the space; characterised by: a compartment wall arranged in the housing so as to partition the space formed by the housing in an axial direction along an outer wall portion of the housing to form two adjacent passages that takes in air vibration generated by the vibration source; a partition wall arranged so as to respectively further partition the adjacent passages into an inner space adjacent the outer surface of the vibration source and an outer space remote from the outer surface of the vibration source, wherein each outer space is respectively provided with an inlet communicating with the inner space, with a first partition member forming an end portion of the passage, and with a second partition member serving as resistance against the air vibration, and closing a downstream side of the passage with respect to a propagation direction of the air vibration in an acoustic portion configured as the space from the inlet at the end portion along the direction of the passage to the second partition member, wherein the inlet and the first and second partition members (5a,5b) are arranged such that the propagation directions of the air vibration in the adjacent passages are reversed to each other. a compartment wall arranged in the housing so as to partition the space formed by the housing in an axial direction along an outer wall portion of the housing to form two adjacent passages that takes in air vibration generated by the vibration source; a partition wall arranged so as to respectively further partition the adjacent passages into an inner space adjacent the outer surface of the vibration source and an outer space remote from the outer surface of the vibration source, wherein each outer space is respectively provided with an inlet communicating with the inner space, with a first partition member forming an end portion of the passage, and with a second partition member serving as resistance against the air vibration, and closing a downstream side of the passage with respect to a propagation direction of the air vibration in an acoustic portion configured as the space from the inlet at the end portion along the direction of the passage to the second partition member, wherein the inlet and the first and second partition members (5a,5b) are arranged such that the propagation directions of the air vibration in the adjacent passages are reversed to each other.