[en] Conventional on-line isotope separators aim at the separation based on mass number, and their resolution is several hundred. If the resolution can be improved by a hundred times, to tens of thousands, the separation based on not only the mass number A of nuclei but also on the number of charges Z is expected to be possible. If it is possible to determine the position to 1/30 of mass spectrum peak width, the mass measurement of 0.1 MeV accuracy in the vicinity of A = 100 is feasible with the resolution of 30,000. Presently the masses of many short life nuclei are not determined with good accuracy, though they have been determined by the measurement of double mass difference or nuclear reaction energy. Therefore the employment of secondary double focusing mass spectrometer was proposed and produced for trial as the on-line mass spectrograph. This apparatus is composed of the combination of cylindrical electric field, static Q lens, and uniform magnetic field, and is easy to manufacture even if it is comparatively large. The examination with a commercially available ion source for mass spectrometers indicated that the resolution of 30,000 is obtainable at the dispersion tolerance of +-0.01 for aperture angles and ion energies in r and z directions. It seems that the on-line mass spectrograph to analyze fission products directly requires 1.5 m or larger electric field radius. The mass determination for considerable number of short life nuclei is supposed to be feasible with the resolution of 20,000 to 30,000. (Wakatsuki, Y.)