PURPOSE: To position and adjust the shadow-masks simply and in a high accuracy, by positioning them through detecting the variation of light quantity passing through the main mask and the submask.

CONSTITUTION: When the light 15 radiated from a light source 11 passes through the opening 70 of a subshadow-mask 30, then through the opening 7 of the main shadow-mask 3, and goes into a light detector 12, an output indicator 14 displays an index value proportional to the quantity of the incident light. The figure a shows the case when the centers of the openings 70 and 7 are not at the same point, and the passing light 20a through he opening 7 has a lack, making the light quantity 21 smaller than the maximum value. On the other hand, the figure b shows the case when the centers of the openings 70 and 7 are almost at the same point, and the passing light 20b through the open ing 7 has no lack, making its quantity 22 maximum. Therefore, if the main shadow-mask 3 is positioned to show a maximum value of the output indicator 14, the centers of openings 70 and 7 are almost coincide. Such a positioning is carried out one by one at the positions selected adequately, and the main shadow-mask 3 is fixed at the position to be the best position as the whole shadow-masks, and welded at the frame.