An outdoor unit of an air-conditioning apparatus comprising: a heat exchanger having a pair of header pipes provided at both ends of the heat exchanger , wherein the heat exchanger includes a plurality of flattened pipes between the pair of the header pipes and communicating with insides of the pair of the header pipes ; a base plate on which the heat exchanger is placed; a first spacer mounted on a lower end portion of each header pipe and covering the lower end portion of each header pipe and a fixing member fixing the heat exchanger via the first spacer , wherein the fixing member is mounted on the first spacer ; wherein the first spacer is made of a resin or a metal that is less noble than a metal forming the heat exchanger , characterized in that the outdoor unit comprises a partition plate which is placed on the base plate and separates a blower chamber and a machine chamber , the heat exchanger includes side plates , wherein a first side plate of the side plates is provided at an outer uppermost flattened pipe of the plurality of flattened pipes and a second side plate of the side plates is provided at an outer lowermost pipe of the plurality of flattened pipes ; a second spacer mounted on a portion of the first and the second side plate and covering the portion of the first and the second side plate is provided, and the heat exchanger is placed on the base plate via the first spacer mounted on the lower end of each header pipe and the second spacer mounted on the second side plate which is provided at the outer lowermost flattened pipe , and a lower end of the heat exchanger is kept horizontal. a heat exchanger having a pair of header pipes provided at both ends of the heat exchanger , wherein the heat exchanger includes a plurality of flattened pipes between the pair of the header pipes and communicating with insides of the pair of the header pipes ; a base plate on which the heat exchanger is placed; a first spacer mounted on a lower end portion of each header pipe and covering the lower end portion of each header pipe and a fixing member fixing the heat exchanger via the first spacer , wherein the fixing member is mounted on the first spacer ; wherein the first spacer is made of a resin or a metal that is less noble than a metal forming the heat exchanger , characterized in that the outdoor unit comprises a partition plate which is placed on the base plate and separates a blower chamber and a machine chamber , the heat exchanger includes side plates , wherein a first side plate of the side plates is provided at an outer uppermost flattened pipe of the plurality of flattened pipes and a second side plate of the side plates is provided at an outer lowermost pipe of the plurality of flattened pipes ; a second spacer mounted on a portion of the first and the second side plate and covering the portion of the first and the second side plate is provided, and the heat exchanger is placed on the base plate via the first spacer mounted on the lower end of each header pipe and the second spacer mounted on the second side plate which is provided at the outer lowermost flattened pipe , and a lower end of the heat exchanger is kept horizontal. <EOS>