With the aspect shown in FIG. 5(a), a bent portion 85b2 is provided with a wide portion 85b3 whose width is larger than that of the other portion of the heating wire 8. The wide portion 85b3 is formed over a predetermined length L in a heating wire that extends from the vertex portion of the bent portion 85b2 in two directions. More specifically, the inner edge portion of the bent portion 85b2 is linear, and a step is formed at the outer edge portion thereof, thus forming the wide portion 85b3. By providing a wide portion at the bent portion 85b2 in this manner, the resistance of the heating wire 8 is smaller than that of the other portion, and thus it is possible to suppress locally high temperatures at the bent portion 85b2. The width of the wide portion 85b3 is larger than the width of the other portion of the heating wire 8, and is preferably 0.05 to 5.0 mm, more preferably 0.3 to 1.0 mm, even more preferably 0.3 to 0.7 mm, and particularly preferably 0.4 to 0.5 mm, for example. Also, the width of the wide portion 85b3 is preferably 1.2 times or more the width of the other portion thereof, more preferably 1.5 times or more, and particularly preferably 2.0 times or more the width of the other portion. On the other hand, the width of the wide portion 85b3 is preferably 3 times or less the width of the other portion thereof.
