An outdoor unit of an air-conditioning apparatus, comprising: a bottom plate having a drain hole formed therein to discharge drain water generated in a heat exchanger; a heater provided on an upper surface of the bottom plate; a heater fixing tool configured 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, wherein the heater fixing tool includes an attachment portion having a screw hole; a screw fastened after inserted into the screw hole from above to fasten 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, wherein: 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; the bottom plate has an upper surface at which a raised portion is formed overlapping with the attachment portion of the heater fixing tool; and the protective cap includes a cover portion configured to cover the tip of the screw and a pair of flange portions (26, 27) formed to extend from opposite sides of an opening edge of the cover portion, and at one of the pair of flange portions (26, 27), a caulked joint is formed to fix the one of the pair of flange portions (26, 27) to the raised portion; and the bottom plate has a lower surface at which a pair of leg portions are formed to support the outdoor unit, and at the pair of leg portions, caulked joints are formed to fix the pair of leg portions to the bottom plate. a bottom plate having a drain hole formed therein to discharge drain water generated in a heat exchanger; a heater provided on an upper surface of the bottom plate; a heater fixing tool configured 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, wherein the heater fixing tool includes an attachment portion having a screw hole; a screw fastened after inserted into the screw hole from above to fasten 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; the bottom plate has an upper surface at which a raised portion is formed overlapping with the attachment portion of the heater fixing tool; and the protective cap includes a cover portion configured to cover the tip of the screw and a pair of flange portions (26, 27) formed to extend from opposite sides of an opening edge of the cover portion, and at one of the pair of flange portions (26, 27), a caulked joint is formed to fix the one of the pair of flange portions (26, 27) to the raised portion; and the bottom plate has a lower surface at which a pair of leg portions are formed to support the outdoor unit, and at the pair of leg portions, caulked joints are formed to fix the pair of leg portions to the bottom plate.