Mucin 5AC (MUC5AC) is one of the main airway mucins implicated in pulmonary diseases with mucous hypersecretion. Neutrophil elastase (NE), a serine protease released by neutrophils, is known to induce MUC5AC synthesis by increasing reactive oxygen species (ROS) generation. Nuclear factor erythroid-2 related factor 2 (Nrf2), a basic-region leucine-zipper transcription factor, is believed to protect against ROS damage by activating a series of antioxidant enzymes; Nrf2 is also reported to reduce NE activity. The aim of our study was to examine the relationship between Nrf2 expression and NE-induced MUC5AC production. We used a small interfering RNA to inhibit Nrf2 expression. The Nrf2 gene and protein expressions were assessed by quantitative real-time polymerase chain reaction, and Western blotting, respectively. The ROS generation was examined using a kit. The expression of MUC5AC was assessed using enzyme-linked immunosorbent assay. The results showed that NCI-H292 epithelial cells, in which the Nrf2 gene expression repressed, were highly predisposed to NE stimulation, with marked exacerbation of ROS generation and reduced secretory leukocyte protease inhibitor production, resulting in high MUC5AC expression. Pretreatment with the potent Nrf2 activator sulforaphane had the reverse effect. These results demonstrate that Nrf2 is a novel nuclear factor involved in down-regulating MUC5AC synthesis by inhibiting ROS generation and augmenting proteinase inhibitor production.