A mass spectrometer with an ionization chamber with a feed channel for a gas to be examined, including an electron source (d, n) for ionizing the gas to be examined, electrodes (c) for accelerating the ionizing electrons, electrodes (g, h, j, m) for the mass-dependent separation of the ions by acceleration/deceleration thereof, a detector (l) for the separated ions, a wiring with metallic conductors. The components are arranged on a plane nonconductive substrate (1), having an energy filter (k) for the ions, the energy filter being embodied as a 90° sector, is constructed in completely planar fashion. The ionization chamber (b), the electrodes (g, h, j, m) for accelerating the electrons and ions, the detector (l) for the ions and the energy filter (k) are produced by a single step of photolithography and etching of a doped semiconductor die (6) applied to the substrate (1) and the wiring (2) and the abovementioned parts are covered by a second flat nonconductive substrate (7).