A second spiral spring is smaller than a first spiral spring and is housed in an open space in the center of the first spiral spring. A folded portion that is formed in an inner end of the first spiral spring and a folded portion that is formed in an outer end of the second spiral spring are hooked in a latch plate, under tension, that is cut and raised from a side. A receiving space is formed between the latch plate and an initial winding portion that starts winding in a curved shape from the inner end of the first spiral spring so as to house a tip of the folded portion of the outer end of the second spiral spring that is hooked in the latch plate.