An outdoor unit of an air-conditioning apparatus, the outdoor unit comprising: a casing constituting a shell of the outdoor unit; an air-sending device chamber disposed in the casing, the air-sending device chamber accommodating a heat exchanger and an air-sending device; the heat exchanger including heat exchanger pipes, protruding toward a machine chamber, and a heat exchanger end plate, through which the heat exchanger pipes extend, disposed on a first end of the heat exchanger; the machine chamber disposed in the casing, the machine chamber accommodating a compressor; a partition separating the air-sending device chamber from the machine chamber; a heat exchanger cover having an L-shaped cross section to cover part of and surround heat exchanger pipes and including a first flat portion defining a first side wall of the heat exchanger cover and a second flat portion defining a second side wall of the heat exchanger cover, the first flat portion being joined to the partition, the second flat portion being joined to the casing; and an electric box including a bottom plate defining a bottom surface of the electric box, the bottom plate being joined to an upper end of the heat exchanger cover, the casing including a shell panel having an outdoor-air inlet that is open, the heat exchanger cover being positioned to face the outdoor-air inlet, and defining, together with the partition, the heat exchanger, and the shell panel, an air passage communicating with the outdoor-air inlet in the machine chamber, the bottom plate of the electric box having a bottom opening (62, 63, 64, 65) communicating with the air passage, the partition having a communication opening, through which the air-sending device chamber communicates with the machine chamber, in upper part of the partition, the electric box including a first side plate defining a side wall of the electric box, the first side plate having a side vent that is open and positioned to face the communication opening. a casing constituting a shell of the outdoor unit; an air-sending device chamber disposed in the casing, the air-sending device chamber accommodating a heat exchanger and an air-sending device; the heat exchanger including heat exchanger pipes, protruding toward a machine chamber, and a heat exchanger end plate, through which the heat exchanger pipes extend, disposed on a first end of the heat exchanger; the machine chamber disposed in the casing, the machine chamber accommodating a compressor; a partition separating the air-sending device chamber from the machine chamber; a heat exchanger cover having an L-shaped cross section to cover part of and surround heat exchanger pipes and including a first flat portion defining a first side wall of the heat exchanger cover and a second flat portion defining a second side wall of the heat exchanger cover, the first flat portion being joined to the partition, the second flat portion being joined to the casing; and an electric box including a bottom plate defining a bottom surface of the electric box, the bottom plate being joined to an upper end of the heat exchanger cover, the casing including a shell panel having an outdoor-air inlet that is open, the heat exchanger cover being positioned to face the outdoor-air inlet, and defining, together with the partition, the heat exchanger, and the shell panel, an air passage communicating with the outdoor-air inlet in the machine chamber, the bottom plate of the electric box having a bottom opening (62, 63, 64, 65) communicating with the air passage, the partition having a communication opening, through which the air-sending device chamber communicates with the machine chamber, in upper part of the partition, the electric box including a first side plate defining a side wall of the electric box, the first side plate having a side vent that is open and positioned to face the communication opening.