The present invention discloses a vibration reducing structure of an air conditioner including: a casing on which a suction hole and a discharge hole for sucking and discharging air have been formed and in which a heat exchanger for exchanging heat with air has been built; a ventilating device having a ventilating fan for blowing air along the suction hole, the heat exchanger and the discharge hole and a motor for driving the ventilating fan, the ventilating fan and the motor being coupled to the casing by a motor mount; and a vibration preventing unit protruded from the surface of the casing, for preventing vibration generated by the ventilating device from being diffused along the casing. Therefore, the vibration reducing structure of the air conditioner maximizes reduction of vibration by a simple process.