The present invention discloses a built-in type outdoor unit for an air conditioner to provide an efficient installation structure for installing the outdoor unit increased in capacity due to high air conditioning capacity in a built-in type. The built-in type outdoor unit includes a louver frame being fixedly installed on a rectangular space inner wall formed on an outer wall of a building, being divided into a suction area and a discharge area, including a plurality of louver blades in each area, and sucking and discharging air through gaps between the louver blades, an outdoor unit casing being formed in a rectangular parallelepiped shape, being fixedly installed on the inside bottom of the building to contact the louver frame, having its one surface facing the suction area and the discharge area of the louver frame opened and the other surfaces closed, and being divided into a suction unit and a discharge unit corresponding to the suction area and the discharge area of the louver frame, a compressor installed in the suction unit of the outdoor unit casing, for compressing a refrigerant gas supplied from an indoor unit through pipe lines, an air-cooled condenser installed in the suction unit of the outdoor unit casing, for condensing the refrigerant gas from the compressor, and a sirocco cooling fan installed in the discharge unit of the outdoor unit casing, for supplying external air to the air-cooled condenser through the suction area, and discharging heat exchanged air through the discharge area.