Helical protrusion portions are formed on a peripheral side surface face of a bobbin, and a helical element is screwed into the helical grooves formed between the helical protrusion portions. The lower end of the bobbing is screwed onto an attachment fitting provided with a first collar portion at an intermediate portion and a second collar portion at the end. The depth of the helical groove portions becomes gradually shallower toward the leading end of the helical protrusion portions, and the helical protrusion portions formed on the lower end of the bobbin are formed higher, so that the position of the helical element is stabilized. Thus, deviation of the resonance point of the helical element can be prevented.