A particle shielding element of a shape memory metal is installed between an electroconductive layer on the inner peripheral surface of a neck glass tube of a cathode-ray tube and a shield cup of an electron gun therein to close a gap between the neck glass tube and the shield cup for thereby guarding the electronic gun against invasion of particles, Before the particle shielding element is inserted into the neck glass tube, the element is deformed to reduce its outer diameter so that the element can be inserted into the neck glass tube without scratching the electroconductive layer. After the element has been installed in Position in the neck glass tube, the temperature therein is adjusted to cause the element to recovery to its initial shape into dust sealing engagement with the electroconductive layer.