An object of the present disclosure is to provide a core electric wire for a multi-core cable that is superior in flex resistance at low temperature, and a multi-core cable employing the same. A core electric wire for a multi-core cable according to an aspect of the present disclosure comprises a conductor obtained by twisting element wires, and an insulating layer covering the conductor, wherein a linear expansion coefficient C of the insulating layer at from 25° C. to −35° C. is no less than 1×10−5 K−1 and no greater than 2.5×10−4 K−1.