© 2017, The Author(s). The medium-energy particle experiments–ion mass analyzer (MEP-i) was developed for the exploration of energization and radiation in geospace (ERG) mission (Arase), in order to measure the three-dimensional distribution functions of the inner-magnetospheric ions in the medium energy range between 10 and 180 keV/q. The energy, mass, and charge state of each ion are determined by a combination of an electrostatic energy/charge analyzer, a time-of-flight mass/charge analyzer, and energy-sensitive solid-state detectors. This paper describes the instrumentation of the MEP-i, data products, and observation results during a magnetic storm.