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 (51, 52) 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 (51, 52) 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.