An outdoor unit of an air-conditioning apparatus includes a fan chamber, a machine chamber, a partition plate, and an electrical component unit. The partition plate is provided, in its upper part, with a planar receiving surface portion for receiving a holder. The holder is provided with a positioning protrusion protruding downward. A cutout portion into which the positioning protrusion is inserted is formed in the receiving surface portion. At least one inclined surface portion that is inclined downward from the receiving surface portion and guides the positioning protrusion to the cutout portion is provided at an end portion of the receiving surface portion that leads to the cutout portion.