Disclosed is a foamed electrical wire with a good breakdown voltage and a production method for same. A foam insulation layer (2) is made from a heat-resistant thermoplastic resin and has an average bubble diameter of 5 [mu]m max. It is desirable if the effective dielectric constant of the foam insulation layer (2) is 2.5 max., and also if the foam insulation layer (2) is made from one of polyphenylene sulfide, polyethylene naphthalate, polyethylene terephthalate, polyetheretherketone, or thermoplastic polyimide. It is even more desirable if said foam insulation layer is made from crystalline heat-resistant thermoplastic resin. It is also desirable if the foamed electrical wire has an external skin layer, which is not foamed, on the outside of the foam insulation layer (2), an internal skin layer, which is not foamed, on the inside of the foam insulation layer (2), or both.