A prefocus lens section in an electron gun assembly includes a second grid that is disposed on an electron beam generating section side, a third grid that is disposed on a main lens section side, and a shield grid that is disposed between the second grid and the third grid. The shield grid is a cup-shaped electrode with a side wall that surrounds an outer peripheral part of the third grid, which part is located on the second grid side, and extends in parallel to a tube axis. The shield grid has a bottom surface disposed to face the second grid and has an open end disposed to face the third grid. A relationship, EcVfEr, is established, where Ec is a potential applied to the second grid, Vf is a potential applied to the shield grid, and Er is a potential applied to the third grid.