A core electric wire for a multi-core cable according to an aspect of the present invention comprises a conductor obtained by twisting element wires, and an insulating layer covering the conductor, a principal component of the insulating layer being a copolymer of ethylene and an α-olefin having a carbonyl group; the α-olefin content in the copolymer being 14% to 46% by mass; and a mathematical product C*E being 0.01 to 0.9, wherein C is a linear expansion coefficient of the insulating layer at from 25° C. to −35° C., and E is a modulus of elasticity thereof at −35° C. Average area of the conductor in the transverse cross section is 1.0 to 3.0 mm2. Average diameter of the element wires in the conductor is 40 to 100 μm, and number of the element wires is 196 to 2,450.