To ensure maintenance workability of devices in a casing and reduce ventilation resistance in the casing while sufficiently ensuring volume of an electrical component box in an air conditioner outdoor unit.

The air conditioner outdoor unit comprises: a heat exchanger corresponding to four lateral faces of a casing excluding one corner; a blower fan located in an upper part of the casing; a ventilation member surrounding an outer periphery of the blower fan; and an electrical component box provided at the corner which is an opening between side ends of the heat exchanger. The electrical component box is arranged so that it overlaps with the ventilation member as viewed from a side, and includes a first part 51 having an inclined wall 75B1 which substantially inclines along the outer periphery of the ventilation member, a second part 52 which is located below the first part 51, is arranged in a position where it does not overlap with the ventilation member as viewed from the side, and is larger in a cross sectional area than the first part 51, and a third part 53 where a cross-sectional shape gradually changes between the first part 51 and the second part 52.