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 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 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 . <EOS>