A horizontal electrophoresis apparatus according to the present invention comprises a first electrode and a second electrode arranged at respective sides of an electrophoretic gel and each in close contact with one side of the electrophoretic gel. By using the first electrode and the second electrode each in close contact with one side of the electrophoretic gel, the horizontal electrophoresis apparatus enables electric current to flow across the entire cross section of the electrophoretic gel, and thus can prevent an electric current difference from occurring between the upper and lower ends of the electrophoretic gel and solve the problem of a reduction in the resolution of protein bands.