PURPOSE: To secure the safety of a worker and to execute efficient work by automatically performing the injection of a gel electrolyte in a gel electrophoretic plate by utilizing pressure.

CONSTITUTION: A migration plate 9 is formed by bonding two glass or guartz plates so as to provide a predetermined gap and the lower end part thereof is vertically inserted and immersed in a container 1 filled with a gel electrolyte 3. The gel electrolyte 3 is supplied to the container 1 from a supply pipe 5. After the container 1 is hermetically closed, the container 1 is internally pressurized to raise the electrolyte through the gap of the bonded plates from the lower end part thereof to the upper end part thereof to make it possible to fill the gap with the electrolyte. Compressed air is sent in the container 1 from a valve 17 by a pipe 13 and controlled so that pressure is held to a predetermined value by a pressure gauge 19. By this constitution, safety and efficiency are secured because it is unnecessary that a worker uses his hand during manufacturing and he can perform separate work.