To improve the heat resistance at a high temperature by forming an insulating layer covering the periphery of a flat rectangular conductor or an intermediate insulating layer with a polymer of polybenzimidazole (PBI).

This flat rectangular electric wire 10 is constituted of a conductor 11 having a substantially rectangular cross section and an insulating layer 12 made of a PBI polymer and covering its periphery. The insulating layer 12 can withstand a high temperature of 350°C or above, and it has excellent heat resistance. When the electric wire 10 is manufactured, if the periphery of the conductor 11 is coated with PBI varnish and baked to form the insulating layer 12 of the PBI polymer after a round conductor is rolled to form the conductor 11 having the rectangular cross section, the insulating layer 12 is not uniformly formed, and the insulating layer 12 at the corner sections of the conductor 11 is made thin. The periphery is coated with the PBI varnish and baked in the fore-process for rolling the round conductor to prevent this phenomenon, and the insulating layer 12 is satisfactorily formed. The insulating layer 12 is made a uniform layer, it is excellent in voltage resistance, and it can be manufactured at a low cost.