To provide an outdoor unit of an air conditioner capable of suppressing the vibration of a partitioning plate by a simple constitution, consequently suppressing the generation of noise accompanied by the vibration.

The inside of a unit case 11 is divided into a heat exchange chamber R1 and a machine chamber R2 by a partitioning plate 16, a heat exchanger 21 and blowers 22A, 22B are mounted in the heat exchange chamber R1, a compressor 31 and an accumulator 32 are mounted in the machine chamber R2, a reinforcing plate 50 is provided at the partitioning plate 16, the accumulator 32 is mounted to the partitioning plate 16 through the reinforcing plate 50, and the back face 16D of the partitioning plate 16 is fixed to a tube plate 35 of the heat exchanger 21 together with the reinforcing plate 50.