To provide an assembling method of an electrode of an electron gun which depends on a circumference form of the electrode, without utilizing the opening part of its-own electrode as a standard position.

The assembling method of the electron gun in a cathode-ray tube consists of a loading stage of the electrode 20; an adjusting stage of the distance between the electrodes along with the vertical axis of the gun; and the loading of the electrode between two jaws 30, 30', 31, and 31' of a positioning tool. Furthermore, it includes positioning of the electrode in a plane perpendicular to the vertical axis by clamping the jaws of the tool in the single direction, until pressure acts on each four angles of the electrode; and maintaining by hot insertion of glass beads to nails aligned in the circumference of the opening part of the electrode of the positioning tool. The tool maintains the electrode at the plane perpendicular to vertical axis of the gun. The jaws have outlines, which aim at cooperating together with the forms of the angles of the electrode, like a technique of closing of the jaws, in which the electrode is brought to a nominal position of the electrode in the perpendicular plane to the vertical axis.