To automatically perform the bending processing work at front and rear two parts of a seal material of a hoop material 1 with the seal material 2 being attached to its end by a simple structure.

A die 37 for V-shape bending has a stopper face 34 and an inverted V-shaped recess 36, and turnable around a rotary shaft 41. When the die 37 for V-shape bending is turned in the direction of an arrow mark, a portion of the hoop material 1 butted to the stopper face 34 close to a fore end of a low-profile projecting member 38 is curved upwardly in a projecting shape, and follows an inner surface of a part 36b in a vicinity of a rear edge of the inverted V-shaped recess 36. When the die is further turned, the part is further pushed bent downwards with the fore end of the substantially low-profile projecting member 38 being a base point. When the die is still further turned, the fore end of the low-profile projecting member 38 is further inserted into the inverted V-shaped recess 36, and a part in a vicinity of the fore end of the hoop material 1 is folded back substantially in a V-shape by the low-profile projecting member 38 and the inverted V-shaped recess 36. After evacuating the die 37 for V-shape bending, a grooved cylindrical body (a die for obtuse angle bending) 39 is turned to perform the obtuse angle bending at the position behind the seal material 2.