An outdoor unit of an air-conditioning apparatus, which is provided with a heat exchanger, an outdoor fan, a compressor, and an electrical component that are provided in a casing of the outdoor unit, the outdoor unit comprising: terminal blocks (10a, 10b, 10c, 10d, 13) to be connected to the electrical component; and a terminal-block mounting portion provided to face from an inside of the casing toward an outside of the casing, wherein the terminal blocks (10a, 10b, 10c, 10d, 13) are arranged in a height direction of the terminal-block mounting portion, and include a first terminal block (10a, 10b, 10c, 10d) located on a lower side of the terminal-block mounting portion, and a second terminal block located on an upper side of the terminal-block mounting portion, wherein the second terminal block is mounted on a terminal-block seat provided on the terminal-block mounting portion, wherein the terminal-block seat includes a mounting portion on which the second terminal block is mounted, and a fixed portion that is in contact with the terminal-block mounting portion, and wherein the fixed portion is smaller in area than the mounting portion, characterized in that the mounting portion protrudes from the fixed portion toward the first terminal block (10a, 10b, 10c, 10d), and in a sectional structure, a space surrounded by the mounting portion, one of sides of the second terminal block that is closest to the first terminal block (10a, 10b, 10c, 10d), the terminal-block mounting portion, and one of sides of the first terminal block (10a, 10b, 10c, 10d) that faces the terminal-block seat is provided. terminal blocks (10a, 10b, 10c, 10d, 13) to be connected to the electrical component; and a terminal-block mounting portion provided to face from an inside of the casing toward an outside of the casing, wherein the terminal blocks (10a, 10b, 10c, 10d, 13) are arranged in a height direction of the terminal-block mounting portion, and include a first terminal block (10a, 10b, 10c, 10d) located on a lower side of the terminal-block mounting portion, and a second terminal block located on an upper side of the terminal-block mounting portion, a first terminal block (10a, 10b, 10c, 10d) located on a lower side of the terminal-block mounting portion, and a second terminal block located on an upper side of the terminal-block mounting portion, wherein the second terminal block is mounted on a terminal-block seat provided on the terminal-block mounting portion, wherein the terminal-block seat includes a mounting portion on which the second terminal block is mounted, and a fixed portion that is in contact with the terminal-block mounting portion, and a mounting portion on which the second terminal block is mounted, and a fixed portion that is in contact with the terminal-block mounting portion, and wherein the fixed portion is smaller in area than the mounting portion, characterized in that the mounting portion protrudes from the fixed portion toward the first terminal block (10a, 10b, 10c, 10d), and in a sectional structure, a space surrounded by the mounting portion, one of sides of the second terminal block that is closest to the first terminal block (10a, 10b, 10c, 10d), the terminal-block mounting portion, and one of sides of the first terminal block (10a, 10b, 10c, 10d) that faces the terminal-block seat is provided.