Separation of ionic or isotopic species in a magnetized sheet plasma has been studied by using the ion cyclotron resonance method. The sheet plasma was used instead of a cylindrical plasma in which many unfavorable collisions take place. An rf electric field is applied to the plasma by two parallel plate electrodes. It is shown that a resonance is strongly dependent on the plasma space potential across the sheet plasma. The resonance spectra of argon, neon, and nitrogen ions in the low dense sheet plasma with a shallow potential well are clearly observed, while the resonances in high dense plasma with a deep potential well are not observed. It is also found that the resonance frequency for a corresponding ion is proportional to the magnetic field strength.