This paper describes a new method for synthesizing massive rare-earth hydrides with high hydrogen contents. The method can substantially reduce the time for synthesizing rare-earth hydrides compared to a conventional method. Thermally decomposed gas from a hydride was used to synthesize another hydride under very high pressure. AlH3 and MgH2 were used as hydrogen sources. When AlH3 was used as a solid source of hydrogen, SmH2.95 was obtained under the pressure of 4.0 GPa at 300 °C in 30 min. When MgH2 was used, SmH2.95 was obtained under 3.0 GPa at 250 °C for 1 h, or under 4.0 GPa at 200 °C for 1.5 h.