Cu-NMR study under high-field up to 17 T has been performed on the three-dimensional spin dimer system NH4CuCl3, which shows the two-stepped magnetization plateaus. The local magnetization of Cu-site measured by NMR showed a significant deviation from the macroscopic magnetization at low temperatures, suggesting the localization of the field-induced triplons. In the high field region around 16 T, two inequivalent Cu sites are observed. Only one of the two sites shows a split in the resonance line below TN ≃ 3.2 K, indicating the existence of the field-induced magnetic order, to which a part of spins in the system participate.
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