  An anomalously high valence state sometimes shows up in transition-metal
oxide compounds. In such systems, holes tend to occupy mainly the ligand $p$
orbitals, giving rise to interesting physical properties such as
superconductivity in cuprates and rich magnetic phases in ferrates. However, no
one has ever observed the distribution of ligand holes in real space. Here, we
report a successful observation of the spatial distribution of valence
electrons in cubic perovskite SrFeO$_3$ by high-energy X-ray diffraction
experiments and precise electron density analysis using a core differential
Fourier synthesis method. A real-space picture of ligand holes formed by the
orbital hybridization of Fe 3$d$ and O 2$p$ is revealed. The anomalous valence
state in Fe is attributed to the considerable contribution of the ligand hole,
which is related to the metallic nature and the absence of Jahn-Teller
distortions in this system.
