An undersea cable installation structure that is installed undersea, comprising: a pair of first cables, wherein each of the first cables comprises at least one power cable, armoring portions (23a, 23b) and an external impermeable layer provided on the periphery side of the armoring portions (23a, 23b) of the respective first cable and formed by winding a tape-shaped component, wherein each power cable of said pair of first cables includes a conductor on which an insulating layer, a shielding layer, an impermeable layer formed by winding a tape-shaped component, and an anti-corrosive layer that are formed in said sequence; a pair of floating installations, wherein an end of a respective one of the pair of first cables is connected to the respective floating installation; characterized by the undersea cable installation structure further comprising: a second cable comprising at least one power cable, an armoring portion and an external impermeable layer provided on the periphery side of the armoring portion of the second cable and formed of a lead sheath, wherein each power cable of the second cable includes a conductor on which an insulating layer, a shielding layer, an impermeable layer formed by a lead sheath, and an anti-corrosive layer that are formed in said sequence; wherein the armoring portions (23a, 23b) of each of the pair of first cables and the armoring portion of the second cable are formed by disposing a plurality of wire material on the periphery side of the whole of a plurality of the power cables in the circumferential direction of the periphery of the whole power cables and providing the wire materials helically in the axial direction of the power cables; wherein the second cable is placed between a pair of the first cables to be connected thereto and is positioned on the seabed over the full length thereof. a pair of first cables, wherein each of the first cables comprises at least one power cable, armoring portions (23a, 23b) and an external impermeable layer provided on the periphery side of the armoring portions (23a, 23b) of the respective first cable and formed by winding a tape-shaped component, wherein each power cable of said pair of first cables includes a conductor on which an insulating layer, a shielding layer, an impermeable layer formed by winding a tape-shaped component, and an anti-corrosive layer that are formed in said sequence; a pair of floating installations, wherein an end of a respective one of the pair of first cables is connected to the respective floating installation; characterized by the undersea cable installation structure further comprising: a second cable comprising at least one power cable, an armoring portion and an external impermeable layer provided on the periphery side of the armoring portion of the second cable and formed of a lead sheath, wherein each power cable of the second cable includes a conductor on which an insulating layer, a shielding layer, an impermeable layer formed by a lead sheath, and an anti-corrosive layer that are formed in said sequence; wherein the armoring portions (23a, 23b) of each of the pair of first cables and the armoring portion of the second cable are formed by disposing a plurality of wire material on the periphery side of the whole of a plurality of the power cables in the circumferential direction of the periphery of the whole power cables and providing the wire materials helically in the axial direction of the power cables; wherein the second cable is placed between a pair of the first cables to be connected thereto and is positioned on the seabed over the full length thereof. a second cable comprising at least one power cable, an armoring portion and an external impermeable layer provided on the periphery side of the armoring portion of the second cable and formed of a lead sheath, wherein each power cable of the second cable includes a conductor on which an insulating layer, a shielding layer, an impermeable layer formed by a lead sheath, and an anti-corrosive layer that are formed in said sequence; wherein the armoring portions (23a, 23b) of each of the pair of first cables and the armoring portion of the second cable are formed by disposing a plurality of wire material on the periphery side of the whole of a plurality of the power cables in the circumferential direction of the periphery of the whole power cables and providing the wire materials helically in the axial direction of the power cables; wherein the second cable is placed between a pair of the first cables to be connected thereto and is positioned on the seabed over the full length thereof.