An outdoor unit of an air-conditioning apparatus includes: a bottom plate having a drain hole formed therein to discharge drain water generated in a heat exchanger; a heater provided at an upper surface of the bottom plate; a heater fixing tool provided to fix the heater to the bottom plate, with the heater interposed between the heater fixing tool and the upper surface of the bottom plate; a screw fastened after inserted into a screw hole from above to fix the heater fixing tool to the bottom plate, the screw hole being provided in overlapping portions of the heater fixing tool and the bottom plate; and a protective cap configured to cover a tip of the screw from below, which projects downwards from a lower surface of the bottom plate. The screw hole is located in a surrounding area which surrounds the drain hole, and the heater fixing tool is fixed to the bottom plate in the surrounding area.