A holder for a capillary cassette closes a chamber, and is fixed to a detection side holder fixing member. An acidic solution container, an alkaline solution container, a pure water container and a drain container are arranged on a reservoir stage having a dry chamber. A holder up/down mechanism and a stage moving mechanism successively bring an end of a capillary array into contact with an acidic solution, an alkaline solution, pure water and nitrogen gas, and the chamber is decompressed for successively introducing these into capillary columns and performing pretreatment. Thereafter a gel container is arranged in the chamber, which in turn is pressurized for charging the capillary columns with a gel.