Abstract: A color cathode ray tube has excellent focusing characteristics over the whole screen thereof, as well as a total length which is short, thus providing a compact monitoring device. The Cathode ray tube has an electron gun in which the ratio between the diagonal size De of a phosphor screen and the distance Lg from the center of the phosphor screen to an end portion of a focusing electrode which forms a main lens portion of the electron gun and faces an anode electrode in an opposed manner is set to De/Lg>1.5. The focusing electrode and the anode electrode have single opening portions whose opening size in the horizontal direction is not less than 68% of the outer diameter of a neck portion and end surfaces of the opening portions face each other in an opposed manner in the tube axis direction along which the electron beams pass.