An outdoor unit of an air conditioner, the outdoor unit having: a parallelepiped casing, wherein an inside of said casing is divided by a partition plate extending between a bottom plate (lh) and a top plate of the casing into: a blower chamber, housing therein a heat exchanger and a blower fan, and a machinery chamber, housing therein a compressor and an electrical component box, wherein the partition plate comprises: an upper partition plate, and a lower partition plate, wherein the lower partition plate is bent so as to protrude toward the blower chamber, characterized in that a receiving part is formed respectively at an upper end surface of the lower partition plate and bent toward the machinery chamber; the electrical component box comprises: the upper partition plate; a front plate that covers a front side of the electrical component box; and a box base plate that receives the upper partition plate and the front plate and is fixed to the lower partition plate, wherein the box base plate is provided with a flange in a periphery thereof, wherein the flange is bent downward, wherein part of the flange is shaped so as to be engaged with the receiving part, and wherein the electrical component box is set to the machinery chamber such that the part of the flange is fitted to the lower partition plate, wherein the flange is shaped so as to be similar to the shape of the upper end surface of the lower partition plate, and the box base plate is positioned with respect to the receiving part of the lower partition plate along the flange of the box base plate, whereby the electrical component box is put on the lower partition plate to be fixed thereto. a parallelepiped casing, wherein an inside of said casing is divided by a partition plate extending between a bottom plate (lh) and a top plate of the casing into: a blower chamber, housing therein a heat exchanger and a blower fan, and a machinery chamber, housing therein a compressor and an electrical component box, wherein the partition plate comprises: an upper partition plate, and a lower partition plate, wherein the lower partition plate is bent so as to protrude toward the blower chamber, characterized in that a receiving part is formed respectively at an upper end surface of the lower partition plate and bent toward the machinery chamber; the electrical component box comprises: the upper partition plate; a front plate that covers a front side of the electrical component box; and a box base plate that receives the upper partition plate and the front plate and is fixed to the lower partition plate, wherein the box base plate is provided with a flange in a periphery thereof, wherein the flange is bent downward, wherein part of the flange is shaped so as to be engaged with the receiving part, and wherein the electrical component box is set to the machinery chamber such that the part of the flange is fitted to the lower partition plate, wherein the flange is shaped so as to be similar to the shape of the upper end surface of the lower partition plate, and the box base plate is positioned with respect to the receiving part of the lower partition plate along the flange of the box base plate, whereby the electrical component box is put on the lower partition plate to be fixed thereto. a blower chamber, housing therein a heat exchanger and a blower fan, and a machinery chamber, housing therein a compressor and an electrical component box, wherein the partition plate comprises: an upper partition plate, and a lower partition plate, wherein the lower partition plate is bent so as to protrude toward the blower chamber, characterized in that a receiving part is formed respectively at an upper end surface of the lower partition plate and bent toward the machinery chamber; an upper partition plate, and a lower partition plate, wherein the lower partition plate is bent so as to protrude toward the blower chamber, characterized in that a receiving part is formed respectively at an upper end surface of the lower partition plate and bent toward the machinery chamber; the electrical component box comprises: the upper partition plate; a front plate that covers a front side of the electrical component box; and a box base plate that receives the upper partition plate and the front plate and is fixed to the lower partition plate, the upper partition plate; a front plate that covers a front side of the electrical component box; and a box base plate that receives the upper partition plate and the front plate and is fixed to the lower partition plate,