To automatically form the tack of a curtain by generating a tack loop bent part on a curtain cloth, generating a number of tack loops having a sized seam on the bent base part, colliding the bent top part of the tack loop to the seam to generate a tack material having a tack mountain.

In the formation of a tack B, the number of tacks B to be required, and the dimension of a tack bent part B are determined. The position to provide the tack bent part D is then determined, and the position is marked, and a sewing corresponding to the dimension of the tack bent part B is performed to generate a tack loop G, and a sized seam H is formed. Such tack loop G are formed on the whole of a curtain cloth A, the loop is wound on a tack knife, and a tack folding knife is advanced and inserted between the tack knives. The loop G is then bent, the contact top end part is collided to the seam H to generate a tack mountain B1, and a tack stopping seam B4 is formed, whereby the tack B is formed.