Magnetic, transport, and x-ray diffraction measurements on single-crystal CeRhAs, the so-called Kondo insulator, have revealed successive transitions at T1=370, T2=235, and T3=165 K. Below T1, the unit cell is doubled along the b and c axes. Thereby, the resistivity jumps upwards and magnetic susceptibility in all the directions drops. Superlattice reflections at (0,1/3,1/3) and (1/3,0,0) appear at T,T2. The latter suddenly increases below T3 where a transport gap is enhanced. These observations indicate that the gap formation is intimately related to lattice modulations.