To form a square wire having a thin diameter, which is used for coils or the like at a low cost, and to compose square slits easily and preciously for forming the square wire.

The apparatus has four rollers 1-4, which are arranged in a fylfot structure by crossing alternately for composing a square slit 5, and a controller for controlling the positions of four rollers 1-4. The controller is a wedge-shaped controller. Each of the rollers 1-4 is varied in the axis direction and the horizontal direction according to the wedge-shaped controller capable of controlling the roller positions finely.