A larva-to-pupa metamorphosis is induced by a low or undetectable level of juvenile hormone (JH) during last instar in holometabolous insects. An exogenous application of JH agonist, pyriproxyfen (PYR), inhibited pupal metamorphosis of the beet armyworm, Spodoptera exigua. Last instar larvae of S. exigua exhibited increase of body size at first 3 days along with active feeding behavior. Also, at this period, prothoracic gland increased in size, while corpora allata remained little change. Storage proteins were accumulated in hemolymph plasma from penultimate to last instars, during which two storage protein genes (SeHex and SeSP1) were actively expressed. A Broad-Complex 1 (BRC1) gene of S. exigua (SeBRC1) was partially cloned and showed a specific expression at the last instar in all tested tissues including hemocytes, fat body, epidermis, gut, nerve, and salivary gland. Knockdown of SeBRC1 expression by its specific double-strand RNA mimicked the antimetamorphic effect induced by PYR treatment. PYR treatment at early last instar inhibited expression of SeBRC1, but did not that of other nuclear receptor, βFTZ-F1. These results indicate that a transcriptional factor, SeBRC1, plays a crucial role in pupal metamorphosis of S. exigua.