  A dedicated isochronous storage ring, named the Rare-RI Ring, was constructed
at the RI Beam Factory of RIKEN, aiming at precision mass measurements of
nuclei located in uncharted territories of the nuclear chart. The Rare-RI Ring
employs the isochronous mass spectrometry technique with the goal to achieve a
relative mass precision of $10^{-6}$ within a measurement time of less than 1
ms. The performance of the facility was demonstrated through mass measurements
of neutron-rich nuclei with well-known masses. Velocity or magnetic rigidity is
measured for every particle prior to its injection into the ring, wherein its
revolution time is accurately determined. The latter quantity is used to
determine the mass of the particle, while the former one is needed for
non-isochronicity corrections. Mass precisions on the order of $10^{-5}$ were
achieved in the first commissioning, which demonstrates that Rare-RI Ring is a
powerful tool for mass spectrometry of short-lived nuclei.
