Provided are a micelle composition for nucleic acid delivery using a temperature-sensitive polymer, and a method for producing the micelle. For example, provided is a polyion complex comprising a temperature-sensitive copolymer and a nucleic acid, wherein the temperature-sensitive copolymer comprises a cationic block and a temperature-sensitive block, the cationic block optionally carrying a hydrophilic block linked thereto, and the polyion complex is obtained by mixing the temperature-sensitive copolymer with the nucleic acid under temperature conditions equal to or lower than the lower critical solution temperature (LCST) of the temperature-sensitive copolymer.