Ion Mass Imager (IMI) is a light-weight ion mass composition instrument. IMI measures ions with energies between 10 eV and 35 keV per charge, and has a 360 degrees field-of-view aperture and uses the spacecraft spin to cover almost the full unit sphere to measure three-dimensional distribution o f ions. IMI will be operated together with the other scientific instruments on Planet-B to study the Martian environment with emphasis on the upper atmosphere interaction with the solar wind. IMI can also measure the possible dust ring around Mars, because the instrument has a wide mass range coveri ng a dusty plasma component.