Abstract: An apparatus for assembling and inspecting an electron gun of a cathode ray tube includes a mandrel 5 and a measuring nozzle 2. The mandrel 5 has an outer diameter larger than the electron-beam-transmission hole of the Gm electrode 30. The mandrel 5 abuts against the Gm electrode 30 from G3 electrode 40 side, to seal the electron-beam-transmission hole of the Gm electrode 30. The measuring nozzle 2 has a tip that can be inserted through the electron-beam-transmission holes of the G1 and G2 electrodes from the G1 electrode side, and blows air toward the mandrel 5. A clearance between the G2 and Gm electrodes are measured according to the change in pressure of the blown air. It is possible to measure the clearance between the electrodes required for assembling the electron gun having the Gm electrode with a small electron-beam-transmission hole.