By using an electrophoretic buffer comprising a polymerized polymer micelle formed by the steps comprising dispersing into an aqueous medium a block copolymer represented by HPLS-HPBS-PLZA, wherein HPLS is a hydrophilic polymer segment, HPBS is a hydrophobic polymer segment, and PLZA is a polymerizable group having an ethylenically unsaturated double bond, and polymerizing the block copolymer as a buffer for a capillary electrophoresis or a microchip electrophoresis, pressurizing the sample after introduction at a given pressure for a given time period, and electrophoresing in an electrophoretic electric field, a polymer compound such as DNA can be separated rapidly and in high separation ability.