[en] The objective of this study is to investigate the emission mechanism of radiophotoluminescence (RPL) in the Ag⁺-doped phosphate glass (glass dosimeter), which is now used as the individual radiation dosimeter, because the emission mechanism of RPL in the glass dosimeter was not fully understood. Optical properties such as RPL emission and excitation spectra and change of RPL spectrum as a function of X-ray irradiation dose were measured for commercially available glass dosimeter. In this study, we discuss the emission mechanism of two RPL peaks at about 2.70 eV (460 nm) and 2.21 eV (560 nm), based on the fact that electrons and holes produced by X-ray irradiation are trapped at Ag⁺ ions to produce Ag⁰ and Ag²⁺ ions, respectively, when the Ag⁺-doped phosphate glass is exposed to X-ray. We propose the emission mechanism of RPL peaks at about 2.70 and 2.21 eV, regarding Ag²⁺ and Ag⁰ ions.