Disclosed is a siRNA-encapsulating polymeric micelle complex (a polyion complex) having high monodispersibility and structural stability and excellent in the ability of transporting siRNA into a cell. Further disclosed are a nucleic acid delivery device, a nucleic acid delivery kit, a pharmaceutical composition, and a gene therapy agent, each of which uses the complex. The polyion complex is characterized by comprising: a block copolymer composed of a polyethylene glycol moiety and a polycation moiety having a thiol group as a side chain at the terminus; and siRNA.