Disclosed is an ion trap mass spectrometer improved to obtain a high sensitivity without the lowering of resolution. By fitting a mesh electrode to an aperture (an ion sampling aperture or an ion extracting aperture) made in endcap electrodes constituting an ion trap mass analysis region, a radio frequency electric field in the mass analysis region is not disturbed even if the diameter of the aperture is set to a large value to heighten ion transmission efficiency. By fitting a shield electrode for preventing collision of ions with an insulated ring constituting an outer wall of the mass analysis region, charging up of the insulated ring is prevented to improve stability of detection signals. Furthermore, by arranging a shield member for shielding stray charged particles detouring through the circumference of the mass analysis region to approach an ion detector, generation of noises based on these stray charged particles is prevented.