The crystal structures of low-temperature forms of argyrodite-type superionic conductors, namely Ag7TaS6 II (between approx. 280 K and approx. 170 K) and Ag7NbS6 II (between approx. 280 K and approx. 140 K), analyzed based on powder X-ray diffraction data have been compared with each other. The crystal structure of Ag7TaS6 II could be described on the basis of a space group Pc with the lattice constants a = 7.453, b = 7.403, c = 12.806 Angstrom and beta = 124.62 degrees. In the Rietveld analysis of Ag7NbS6 II, a commensurately modulated structure approach was adopted to obtain a smooth convergence. The crystal data of Ag7NbS6 II are expressed as a(1) = 7.368, a(2) = 14.770, a(3) = 12.768 Angstrom, sigma = (0.25 0 0), beta = 124.28 degrees in a four-dimensional formalism with a superspace-group Pc(alpha 1/2 gamma), that is A = 29.47, B = 14.770, C = 12.768 Angstrom, beta = 124.28 degrees in a three-dimensional formalism with a space group Cc. In Ag7TaS6 II and Ag7NbS6 II, almost a half of Ag ions are in distorted tetrahedrons and the other Ag ions are in triangular faces shared by two tetrahedrons. A split model of mobile Ag ions in the room-temperature phase I is discussed in connection with Ag sites in the low-temperature phase II.