An insulated electric wire includes a conductor and an insulating layer covering an outer periphery of the conductor.The insulating layer is formed of a crosslinked polymer of an insulating resin composition that contains a resin component containing a first copolymer which is a copolymer of ethylene and an unsaturated hydrocarbon having 4 or more carbon atoms and which has a density of less than 0.88 g/cm3, a second copolymer which is a copolymer of ethylene and an unsaturated hydrocarbon having 4 or more carbon atoms and which has a density of 0.88 g/cm3 or more and less than 0.91 g/cm3, and a third copolymer which is a copolymer of ethylene and an acrylic acid ester or a copolymer of ethylene and a methacrylic acid ester, or containing the second copolymer and the third copolymer, in which a content of the second copolymer is 40% by mass or more of a total content of the first copolymer and the second copolymer and 60% by mass or less of a total content of the first copolymer, the second copolymer, and the third copolymer, and a ratio represented by the total content of the first copolymer and the second copolymer: a content of the third copolymer (mass ratio) is 80:20 to 40:60, and 30 to 100 parts by mass of a flame retardant and 1 to 5 parts by mass of a crosslinking assistant relative to 100 parts by mass of the resin component.