A quantum spin magnet NH4CuCl3 is a three dimensional dimer system, which shows a two-stepped magnetization plateaus at M = 1/4 and 3/4. In order to investigate the spatial configuration of field-induced triplons under high-field, Cu-NMR study has been performed in the field region between the first and the second plateau. It has been found that the same numbers of the differently-oriented two dimers in a unit cell remain unoccupied by triplons. This gives a strong restriction to the model to describe the plateaus in this system.