The conversion of ionic valence induced by X-ray irradiation can possibly be applied in X-ray imaging devices. Here, we report the X-ray irradiation time dependence of the valence change of Sm ions doped in a BaSO4 matrix. The samples were prepared by sintering a mixture of BaSO4 and Sm2O3 powder. The as-prepared sample consisted mainly of Sm3+. Sm2+ and Sm3+ were detected by using the photoluminescence (PL) spectral shapes. After X-ray irradiation (lambda = 0.154 nm), the PL intensity of the Sm2+ increased proportionally to the X-ray irradiation time from 4 to 1000 s. The intensity began to saturate at more than 1000 s. The intensity of Sm3+ was constant regardless of the increasing Sm2+ intensity.