The far-infrared reflectance spectra of meltgrown single crystal CdGa2S4 have been measured at room temperature. Nine of ten reflectance spectra which were anticipated from the irreducible representation are observed for this material. These reflectance peaks are attributed to the reststrahlen bands due to the ionic bonds between the positive metal ions and negative chalcogen ions. From the reflectance spectra, the transverse and longitudinal optical phonon frequencies and the static dielectric constant are calculated. In this case, ωTO=84 cm-1, 134 cm-1, 166 cm-1, 239 cm-1, 265 cm-1, 310 cm-1, 319 cm-1, 342 cm-1 and 359 cm-1, ωLO=89 cm-1, 136 cm-1, 171 cm-1, 245 cm-1, 267 cm-1, 312 cm-1, 340 cm-1, 348 cm-1 and 384 cm-1, and ε0=9.14 for CdGa2S4 single crystal.