PURPOSE: To obtain an electrophoretic device of high resolving power which has an electrophoretic path of a length being equal to or larger than the length of an electrophoresis supporting flat plate on one electrophoretic plate.

CONSTITUTION: Thin grooves each constructed of a linear part and a curved part are formed in the surface of one electrophoresis supporting flat plate 1, the other electrophoresis supporting flat plate 2 is stuck fast thereon, thin tubes thus formed are used as electrophoretic paths and thereby an electrophoretic plate 1a having the electrophoretic paths 3 longer than the dimension of the long side of the electrophoresis supporting flat plate 1 is formed. A groove 5 for applicating a laser beam 6 is so provided as to intersect the electrophoretic paths 3 and fluorescence from a sample making electrophoresis is detected. The electrophoretic paths 3 in a plurality each having a length equal to or larger than the dimension of one side of the electrophoretic plate can be formed on one electrophoretic plate and separation of DNAs of a 1000 base length and a 1001 base length is enabled. As the result, a processing speed can be increased sharply in determination of disposition of massive DNAs represented by human genes, or the like.