In this article, we report a simple and rapid immunoassay based on microscale electrophoresis using a microchip equipped with a reagent-release cartridge at the sample reservoir. The cartridge consists of an outer tube, a paper filter retaining dried reagents and an antibody, and inner tubes to fix the paper filter. When a sample solution containing an antigen is introduced into the sample reservoir through the reagent-release cartridge, the reagents and the antibody retained on the filter dissolve immediately into the sample solution. The sample solution and reagents containing the antibody are then mixed, promoting rapid immunoreaction between the antigen and the antibody. After sample introduction into the reservoir, the antigen-antibody complex is rapidly separated from the free antibody by microchip zone electrophoresis. The immunoassays for bovine serum albumin and human IgG were easily performed merely by pouring the sample solution into the cartridge in the newly developed device followed by microchip electrophoresis, resulting in the rapid and clear separation of the immunocomplexes from the free antibodies. These results showed that the developed device was applicable to a rapid and simple immunoassay.