An outdoor unit of an air conditioner, comprising: a case comprising: a base pan configured to define a bottom surface; an out panel coupled to be upright at an edge of the base pan; and a cover plate seated on an upper surface of the out panel and having a discharge port ; a control box accommodated in the case and installed at one side of a rear surface of the out panel; a heat exchanger accommodated in the case; a fan motor assembly accommodated in the case; and a compressor seated on the base pan, wherein the control box comprises: a housing comprising left and right side surfaces , an upper surface and a rear surface , and of which a front surface and a lower surface are open; and a plurality of electrical components which are accommodated in the housing, wherein the upper surface of the control box is formed to be inclined downward from a front end thereof toward a rear end thereof, characterized in that a stepped portion extends further upward from the upper surface of the housing, and a plurality of outlet ports are formed at the stepped portion , the outdoor unit further comprises a blocking rib which protrudes from a portion on the upper surface of the control box and extends in a widthwise direction, to block rain water so that it does not flow to the plurality of electrical components , and the portion on the upper surface of the control box corresponds to a position between the stepped portion and the front end of the upper surface of the control box. a case comprising: a base pan configured to define a bottom surface; an out panel coupled to be upright at an edge of the base pan; and a cover plate seated on an upper surface of the out panel and having a discharge port ; a base pan configured to define a bottom surface; an out panel coupled to be upright at an edge of the base pan; and a cover plate seated on an upper surface of the out panel and having a discharge port ; a control box accommodated in the case and installed at one side of a rear surface of the out panel; a heat exchanger accommodated in the case; a fan motor assembly accommodated in the case; and a compressor seated on the base pan, wherein the control box comprises: a housing comprising left and right side surfaces , an upper surface and a rear surface , and of which a front surface and a lower surface are open; and a plurality of electrical components which are accommodated in the housing, a housing comprising left and right side surfaces , an upper surface and a rear surface , and of which a front surface and a lower surface are open; and a plurality of electrical components which are accommodated in the housing, wherein the upper surface of the control box is formed to be inclined downward from a front end thereof toward a rear end thereof, characterized in that a stepped portion extends further upward from the upper surface of the housing, and a plurality of outlet ports are formed at the stepped portion , the outdoor unit further comprises a blocking rib which protrudes from a portion on the upper surface of the control box and extends in a widthwise direction, to block rain water so that it does not flow to the plurality of electrical components , and the portion on the upper surface of the control box corresponds to a position between the stepped portion and the front end of the upper surface of the control box. <EOS>