To prevent the problem that the straight line portion of the zigzag shape of a nichrome foil hangs downward or is deformed in a shape of waving in the vertical direction due to extension of the nichrome foil, or the mount part formed in a sectional nearly mountain shape is bent backward.

Extension restricting members 501-510 that restrict extension in the width direction of the nichrome plates 42 are arranged in a row at nearly central position of the heater unit plate 20 and the lower end portions 42b of the nichrome plates 42 that form a zigzag shape are inserted and engaged in the recesses that are constructed by the extension restricting members 501-510, thereby, the extension of the nichrome plates 42 in the width direction at the time of heating is restricted. Hence, it becomes possible to maintain the shape of the mount part 42a of the sectional nearly mountain shape that is applied to the nichrome plate 42 for improving strength and to prevent hanging downward or bending backward of the nichrome plates 42.