PURPOSE: To supplement under exposure owing to an imperfect slot at the periphery at the longer side of a fluorescent screen, and to improve the form of the periphery, by increasing the designation of the effective diameter in the scope of the bridge width between slots in the longitudinal direction.

CONSTITUTION: The fluorescent screen of a color cathode-ray tube has a form of a roughly slender rectangle and consists of two long sides, two short sides, and four corners, their curvature radiuses being represented by r1, r2, and r3 respectively. The form of the periphery of the fluorescent screen depends on the peripheral form of a shadow mask which is used for exposure by combining to the screen. The longitudinal effective diameter (e) of the shadow mask is determined instantly from the longitudinal effective diameter of the fluorescent screen. In this case, the longitudinal pitch Pv of longitudinal slot is determined not to generate a moire. Then, an optimum Y-axis diameter is found between (g) and (f) in the scope not to change the actually effective diameter, and by designating peripheral imaginary line in the scope between (i) and (h), the exposure shortage of an imperfect slot is supplemented, and the peripheral form of the long side of the fluorescent screen can be improved.