The present invention provides a siRNA-encapsulated polymer micelle complex (a polyion complex), which has high monodispersibility and structural stability and is excellent in the ability of delivering siRNA into a cell. The present invention further provides a nucleic acid delivery device, a nucleic acid delivery kit, a pharmaceutical composition and a gene therapy agent, each of which comprises the complex. The polyion complex of the present invention is characterized by comprising: a block copolymer constituted by a polyethylene glycol portion and a polycation portion having a side chain whose terminus is a thiol group； and siRNA.
