An outdoor unit of an air conditioner includes a partition plate that separates a machine compartment, in which a compressor unit, a refrigerant pipe, and an electrical-part storage are disposed, and an air blower compartment; an electrical-part storage that is disposed so as to extend from the machine compartment side of the partition plate to the air blower compartment side of the partition plate, and that stores therein an electrical part; an auxiliary board that is disposed in the machine compartment and on which an electrical part that is other than the electrical part installed in the electrical-part storage, is installed; and a board fixing portion that is interposed between the electrical-part storage located in the machine compartment and the auxiliary board to fix the auxiliary board thereto.