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The fine rectangular magnet wire possesses the excellent space factor compared with round wires. The insulation of magnet wires is generally formed by varnish dipping process. However, thus produced rectangular magnet wires may show the relatively low electrical breakdown strength because of the existence of thin layer at the corner parts. The electro-deposition of insulating varnish is available to form the uniform insulation layer even at the corner parts of rectangular conductor. The authors have successfully developed the electro-deposition polyimide insulation which had both high heat-resistance and excellent flexibility. As the results of experiments, it was revealed that this PI insulation film possessed high electrical breakdown strength, because it could cover the conductor very uniformly. Furthermore, it was found that the heat-resistance of this insulation exhibited 230 °C of temperature index in ASTM D2307. These results suggested that this newly developed rectangular magnet wire with polyimide insulation could be adopted to electric devices such as circuit board coils for laptop PC and ultra small sized servomotor, and parts of hybrid electric vehicle (HEV), fuel cell vehicle (FCV), etc.