Cathode-ray tube

(Amended) A cathode ray tube comprising:

• a bulb whose inside is maintained in vacuum;
• an electron gun provided inside of said bulb to eject electron beams;
• a fluorescent material which is provided on the inside wall of said bulb and which emits light when electron beams ejected from said electron gun are applied thereto;
• beam deflection means for deflecting said electron beams so as to scan the surface of said fluorescent material;
• an inner magnetic shield provided inside of said bulb for decreasing the deviation of the trajectory of the electron beams deflected by the beam deflection means in an external magnetic field;
• a shadow mask arranged in front of the inside surface of said fluorescent material;
• and
• a polygon-shaped mask frame for fixing said shadow mask, the frame having a side member constituting each side and a joint portion for joining adjacent side members, wherein the adjacent side members in each joint portion satisfies the following condition;
• μr, 1[≧]≧μr, s≧1
• where μr, 1 represents a relative magnetic permeability of longer side member and μr, s represents a relative magnetic permeability of the other member.