A core wire for multi-core cables includes a conductor obtained by twisting a plurality of elemental wires, and an insulating layer coated on an outer peripheral surface of the conductor. The insulating layer contains polyethylene-based resin as a main component, and the product of a linear expansion coefficient C1 of the insulating layer in the range of 25° C. to −35° C. and an elastic modulus E1 at −35° C., namely (C1×E1), is 0.01 MPaK−1 or more and 0.90 MPaK−1 or less. The melting point of the polyethylene-based resin is 80° C. or higher and 130° C. or lower.