Infrared transmission spectra of the ionic conductors Ag7TaS6, Ag7NbS6 and Ag7-xTaSe6-xIx (0 less than or equal to x less than or equal to 1), which belong to the "argyrodite family" phase, have been studied in the temperature range between 300 and 80 K. As for Ag7TaS6, Ag7NbS6 and Ag7-xTaSe6-xIx (x=0.2, 0.3 and 0.4), the observed M-X (M=Ta, Nb; X=S, (Se, I)) stretching peaks have shown the shifts or the splittings, which suggest the phase transitions, with decreasing temperatures. The results obtained have been interpreted as compared with those of Raman spectroscopic studies and powder X-ray diffraction studies.