We have investigated high-field TF-μSR and ¹H-NMR on the S = 1/2 three dimensional spin dimer system NH₄CuCl₃, which shows a magnetic order at T_N = 1.29 K under zero field and the two-stepped plateaus in high field regions H_C1 = 5.0 T – H_C2 = 12.8 T and H_C3 = 17.9 T – H_C4 = 24.7 T. Both probes showed spectra containing multiple peaks corresponding to different hyperfine fields, demonstrating the existence of magnetically inequivalent dimers in a unit cell. In the slope-state field region H < H_C1, the existence of magnetic order was confirmed by the splitting in some of the NMR peaks. The whole width of NMR and μSR spectra increased with increasing field, and no drastic change in their profile was observed at H_C1. This indicates that the change in the spin state is continuous on entering the plateau state, and that no liquid-solid transition of magnons takes place at the phase boundary.