The laser-heated diamond anvil cell has been widely used to study mineral physics under high pressure and temperature, and these studies have provided valuable information in understanding planetary interiors; however, use of the spectroradiometric method in the studies has raised concerns about the accuracy of obtained temperature values. We have built a laser-heating system coupled with nuclear resonant inelastic x-ray scattering to explore particular physical properties of deep-Earth materials. Energy spectra of iron were measured up to 58 GPa and 1700 K. The detailed balance principle applied to the inelastic x-ray scattering spectra provides absolute temperatures of the laser-heated sample. These temperatures are in very good agreement with values determined from the thermal radiation spectra fitted to the Planck radiation function up to 1700 K. Our data provide, for the first time, independent confirmation of the validity of temperatures determined from spectroradiometric method in the laser-heated diamond cell experiments.