In this investigation, sulfide glass systems m which all of rare-earth elements can be incorporated are explored at first. As a result the Ga_2S_3-GeS_2-La_2S_3-Ln_2S_3 system glasses (Ln=yttrium or lanthanoids) were found. The glass structures were clarified by means of the X-ray radial distribution analysis, Raman spectral measurement, reverse Monte Carlo simulation and EXAFS analysis. Furthermore the characterization of the glasses were performed by measuring various physicochemical properties such as thermal behavior, refractive index, density and Vickers hardness. On the other hand, the optical properties of Er^<3+> in the Ga_2S_3-GeS_2-La_2S_3-Ln_2S_3 glasses were examined by the optical absorption, fluorescence lifetime and up-conversion luminescence measurements. Based on these spectroscopic data and the Judd-Ofelt theory, radiative transition probability, multiphonon relaxation rate, emission quantum efficiency and so on were evaluated. The Er^<3+> up-conversion characteristics of the present sulfide glass were compared with those of fluoride glasses to make clear the possibility of optically functional materials.