While iron occurs predominantly in the form of Fe2+ in the Earth's upper mantle and transition zone, Al-bearing (Mg,Fe)SiO3 perovskite (Pv), a primary mineral in the lower mantle, contains a large proportion of ferric iron (Fe3+). It has been demonstrated that such Fe3+ strongly affects physical and chemical properties of Pv. On the other hand, the iron substitution mechanism and valence state of iron in postperovskite (PPv) are still unclear. Here we determined the valence state of iron in PPv with changing Al3+ content, on the basis of electron energy-loss near-edge structure spectroscopy measurements. The results show that PPv includes a small amount of Fe3+, which is independent from the Al3+ content. This indicates that Fe3+ in Pv and coexisting metallic iron recombine to form Fe2+ upon phase transition from Pv to PPv in pyrolitic mantle. Such Fe2+ partitions preferentially into (Mg,Fe)O ferropericlase (Fp), and therefore PPv is depleted in iron compared to Pv. Such a marked change in the valence state of iron and resulting iron depletion in PPv have broad implications for seismic and transport properties in the lowermost mantle. Phase transition from Pv to Fe-poor PPv occurs in a much narrower pressure range than that for a fixed iron content.