To provide an air conditioner's outdoor unit which allows the other outdoor unit to be laminated on the top surface of a casing, by providing a reinforcing plate along the top surface of the same casing that is composed of a synthetic resin, and also which allows the drain water of the upper outdoor unit to be led to the lower outdoor unit and then drained outside.

When the other outdoor unit is laminated on the top surface of an outdoor unit, a leg 22 is positioned by a positioning means. The first thin part of a casing 1 is crushed to open by a drain member F, which is mounted to a drain hole 9 that is provided at the bottom plate of the upper outdoor unit, and which is equipped with a tubular part Fb corresponding to the through hole of a reinforcing plate. A screw, which is inserted through the second thin part of the casing that is crushed by the leading end of the screw to open, and which is inserted through a through hole 22a, is threadedly fixed to the threaded aperture of the reinforcing plate. The drain water, which is led from the upper outdoor unit through the drain member from the opening part that is formed by crushing the first thin part, is drained outside from the drain hole that is provided at the bottom place of the lower outdoor unit.