Interleukin-6 (IL-6) is a key molecule involved in the pathogenesis of several inflammatory diseases and malignancies. Treatments that inhibit IL-6 mitigate the clinical conditions of such diseases. Here, we report on the development of a new receptor inhibitor of IL-6 (NRI) by genetically engineering tocilizumab, a humanized anti-IL-6 receptor monoclonal antibody which specifically blocks IL-6 signaling. This NRI consists of VH and VL of tocilizumab in a single-chain fragment format dimerized by fusing to the Fc portion of human immunoglobulin G1. The binding activity to IL-6 receptor and the biological activity of the purified NRI were found to be similar to those of parental tocilizumab. Because NRI is encoded on a single gene, it is easily applicable to a gene delivery system using virus vehicles. We administered an adenovirus vector encoding NRI to mouse i.p. and monitored the serum NRI level and growth reduction property on S6B45, an IL-6-dependent multiple myeloma cell line, in vivo. Adequate amount of the serum NRI level to exert anti-IL-6 action could be obtained by the NRI gene introduction combined with adenovirus gene delivery, and this treatment inhibited the in vivo S6B45 cell growth significantly. These findings indicate that NRI is a promising agent applicable to the therapeutic gene delivery approach for IL-6-driven diseases.