A battery protection operation having higher safety is performed in a battery pack. The device elements in which the distributed feedback grating and the distributed Bragg reflector gratings are formed have a lower effective index than the index of the interelement regions and are spaced so as to form an antiguided array. The spool main 7 includes a bobbin trunk 7a around which fishing line is wound, and a larger diameter tubular skirt 7b, formed unitarily with the rear end of the bobbin trunk 7a. At least some of the plurality of transistors may be connected to each other.