To cut down the cost for manufacture and to improve productivity by a method wherein a square-shaped conductor is rolled to longitudinal direction, and the amount of crushing of the edge side positioned outside a coil against the edge side positioned inside the coil is increased gradually.

Rolling rollers 11 and 12 are formed in the same diameter in axial direction respectively, and they are arranged in parallel with each other leaving a preset spacing. The form of cross section and the amount of crushing of a square shaped conductor Wt are set in advance in such a manner that the amount of crushing on the edge side Wo, positioned outside coil in a square shaped conductor Wt, is becoming larger than that of the Wi side positioned inside the coil. A square-shaped conducting wire Wt is passed between the rolling rollers 11 and 12. The square-shaped conducting wire Wt is rolled to longitudinal direction, its cross section becomes rectangular shape, and its shorter side is coincided with the interval between the rolling rollers 11 and 12. The square-shaped conducting wire Wt becomes a round shaped coil by the difference of the crushing amount between the edge side Wi and Wo.