PURPOSE: To improve the strength of grids by providing glass thin plates between each grids and at the specified locations above and below them and sealing the space between glass plates with the use of soldering glass while preset tension is being applied to each grids.

CONSTITUTION: The peripheral section 2 of a three-layered grid is chucked to a mounted movable section 3. Then, at the outside of a grid effective section 4, a glass thin plate 6 in which soldering glass 5 is pre-applied on both surfaces is arranged between grids and the glass thin plate 6 in which the soldering glass 5 is pre-applied on one-side surface makes contact between these grid plates and on these upper and lower surfaces. Subsequently, both ends 7 of the glass thin plate 6 are abutted and combined mutually and the combined section is received by a supporting fixture 8. After that, the upper and lower sides of the glass thin plate 6 are pressed gently and the mounting movable section 3 is moved. Preset tension is applied to the grid 4 and the grid is heated, then the soldering glass is sealed. Thus, the grid is hung with tension. As a result, the strength of the grid is enhanced and the accuracy of the distance between electrodes is also improved.