A submersible motor electric wire includes a conductor including a copper wire and an enamel coating formed on the periphery of the copper wire, and an insulating sheath layer formed outside the conductor. The insulating sheath layer includes a silane graft polymer as a main constituent. The silane graft polymer is formed by a graft-polymerization that an unsaturated silane compound and an organic peroxide are added to polyethylene, and is cross-linked contacting with water in the presence of a siloxane condensation catalyst. Alternatively, the insulating sheath layer includes a composition including polyethylene as a main constituent, and the composition is cross-linked by being irradiated with an ionizing radiation.