To enhance the separation accuracy by enlarging the inside diameter of a capillary tube gradually toward the inlet end part thereof thereby making uniform the field distribution in capillary electrophoresis.

An acute edge part formed at the inlet end 15 of a capillary tube 11 is cut off by chemical etching using hydrofluoric acid during the manufacturing stage and the inlet end 15 is enlarged in trumpet-shape. The surface is then smoothed and finished to have acute inner surfaces 47a, 47b with respect to the end face, if possible. The capillary tube 11 having enlarged terminal 15 provides a uniform field 48 and a plane Y, shown by an Y-Y line, does not intersect with many lines of field strength and a relatively uniform field strength distribution is obtained in the radial direction along the plane Y. An electrophoretic sample is scarcely diffused at the inlet and the fraction thereof has a trend to migrate as a sharp and identifiable band, a sample of small capacity or a sample not available easily can be identified efficiently and separately.