An air conditioner comprising: a base pan, a compressor mounted on a compressor frame, the compressor frame installed on an upper surface of the base pan, a first heat exchanger mounted on the upper surface of the base pan, an outdoor blower unit provided at a left side of the first heat exchanger, a main drain pain installed on a central-rear part of the front frame of the air conditioner to divide an inner space of the air conditioner into upper and lower halves, a second heat exchanger installed on a rear half of the main drain pan, an indoor blower unit installed at an upper side of the main drain pan to guide introduction and discharge of indoor air; and a control box in which a plurality of electronic components for controlling the operation of the air conditioner are installed, the air conditioner characterized in that: the control box is installed vertically across the main drain pan, the upper half of the control box protruding upward from the main drain pan and the lower half protruding downward from the main drain pan, the control box comprising a capacitor exposing hole in which a capacitor is provided to extend downward into the lower half of the air conditioner; wherein the first heat exchanger is installed at the central part of the base pan in the front-rear direction; and wherein the air conditioner further comprises a control box installation opening for receiving the control box and formed at an end of the main drain pan, wherein the control box installation opening is indented inward from an outer surface of the main drain pan. a base pan, a compressor mounted on a compressor frame, the compressor frame installed on an upper surface of the base pan, a first heat exchanger mounted on the upper surface of the base pan, an outdoor blower unit provided at a left side of the first heat exchanger, a main drain pain installed on a central-rear part of the front frame of the air conditioner to divide an inner space of the air conditioner into upper and lower halves, a second heat exchanger installed on a rear half of the main drain pan, an indoor blower unit installed at an upper side of the main drain pan to guide introduction and discharge of indoor air; and a control box in which a plurality of electronic components for controlling the operation of the air conditioner are installed, the air conditioner characterized in that: the control box is installed vertically across the main drain pan, the upper half of the control box protruding upward from the main drain pan and the lower half protruding downward from the main drain pan, the control box comprising a capacitor exposing hole in which a capacitor is provided to extend downward into the lower half of the air conditioner; wherein the first heat exchanger is installed at the central part of the base pan in the front-rear direction; and wherein the air conditioner further comprises a control box installation opening for receiving the control box and formed at an end of the main drain pan, wherein the control box installation opening is indented inward from an outer surface of the main drain pan. the control box is installed vertically across the main drain pan, the upper half of the control box protruding upward from the main drain pan and the lower half protruding downward from the main drain pan, the control box comprising a capacitor exposing hole in which a capacitor is provided to extend downward into the lower half of the air conditioner; wherein the first heat exchanger is installed at the central part of the base pan in the front-rear direction; and wherein the air conditioner further comprises a control box installation opening for receiving the control box and formed at an end of the main drain pan, wherein the control box installation opening is indented inward from an outer surface of the main drain pan.