An outdoor unit of an air conditioning machine in which the inside of a casing is partitioned above and below via a horizontal partition plate, and the upper portion is made to be a heat exchange chamber in which heat exchangers and an air blower are arranged while the lower portion is made to be a machine chamber in which a compressor and the like are arranged, the machine chamber being provided with an electric component box which houses electric components or the like, the outdoor unit further comprising: a heat sink to cool down electric components with a great heating value, the heat sink being projected from a side plate configuring the electric component box; a cooling duct provided along the side plate of the electric component box from which the heat sink is projected, the cooling duct covering the heat sink and comprising openings (12a, 12b) formed in the horizontal partition plate and a base plate (1 a) of the case, and the cooling duct introducing outside air from the opening at the base plate of the case accompanying an operation of the air blower to exchange heat between the outside air and the heat sink, and then guiding the air out of the opening of the horizontal partition plate to the heat exchange chamber wherein two compressors (8a, 8b) are arranged in the machine chamber, the electric components with a great heating value are two inverter devices (C2a, C2b) which drive separately the two compressors (8a, 8b), and the two inverter devices (C2a, C2b) are arranged in a horizontal direction; a vent hole which is formed in the side plate of the electric component box, the vent hole conducting air in the electric component box and guiding the air so as to be discharged into the cooling duct; a partition plate which partitions the side plate of the electric component box in which the vent hole is formed, and the inside of the electric component box; and guide holes formed in the partition plate, the guide holes allowing the inside of the electric component box and the vent hole to be communicated with each other. a heat sink to cool down electric components with a great heating value, the heat sink being projected from a side plate configuring the electric component box; a cooling duct provided along the side plate of the electric component box from which the heat sink is projected, the cooling duct covering the heat sink and comprising openings (12a, 12b) formed in the horizontal partition plate and a base plate (1 a) of the case, and the cooling duct introducing outside air from the opening at the base plate of the case accompanying an operation of the air blower to exchange heat between the outside air and the heat sink, and then guiding the air out of the opening of the horizontal partition plate to the heat exchange chamber wherein two compressors (8a, 8b) are arranged in the machine chamber, the electric components with a great heating value are two inverter devices (C2a, C2b) which drive separately the two compressors (8a, 8b), and the two inverter devices (C2a, C2b) are arranged in a horizontal direction; a vent hole which is formed in the side plate of the electric component box, the vent hole conducting air in the electric component box and guiding the air so as to be discharged into the cooling duct; a partition plate which partitions the side plate of the electric component box in which the vent hole is formed, and the inside of the electric component box; and guide holes formed in the partition plate, the guide holes allowing the inside of the electric component box and the vent hole to be communicated with each other.