To clarify the molecular mechanism of metamorphosis, we analyzed the Broad-Complex (BR-C) gene in the beet armyworm, Spodoptera exigua. We obtained a partial BR-C sequence from a pyrosequencing cDNA library. The BR-C of S. exigua was expressed only in the final larval instar, at which its expression was clearly detected in the epidermis. A treatment of a juvenile hormone analog, pyriproxyfen, inhibited larval-pupal metamorphosis and suppressed the BR-C expression. The hormonal treatment also inhibited expression of two storage protein genes that were usually expressed during a final larval instar. RNA interference of the BR-C using its double strand RNA suppressed BR-C expression and inhibited the larval-pupal metamorphosis. These results suggest that the BR-C is critical to induce larval-pupal metamorphosis of S. exigua.