Electromagnetic ion cyclotron (EMIC) waves are often observed in the magnetosphere, playing an important role in scattering relativistic electrons in the radiation belts. EMIC waves show frequency variations of rising and falling tones. By performing a hybrid code simulation, we have clarified the detailed nonlinear process of the frequency variation when the waves are growing from electromagnetic thermal fluctuations near the magnetic equator. The frequency variation also accounts for formation of subpackets in EMIC rising-tone wave packets.