[en] ⁷⁵As NQR/NMR studies were performed to investigate the successive phase transitions found recently, the gap formation and their interplay in a Kondo semiconductor CeRhAs. NQR/NMR spectra in their respective phases change, reflecting lattice modulation modes, q₁ = (0, 1/2, 1/2), q₂ = (0, 1/3, 1/3) and q₃ = (1/3, 0, 0). In particular for well-resolved three NQR lines corresponding to the q₃ mode in the lowest temperature phase, the nuclear spin-lattice relaxation rate (T₁T)^-1 shows an activation type T-dependence, suggesting a gap opening over the entire Fermi surface, in contrast to the V-shaped gap in isostructural CeNiSn and CeRhSn. The evaluated gap of 272 K and the bandwidth of about 4000 K are one order of magnitude larger than those in CeNiSn and CeRhSb. A lattice modulation forms a gap different from the V-shaped gap. (author)