The Mn4Ca cluster is present as a catalyst in the active site of photosystem II (PS II), which is responsible for water splitting and oxygen production in photosynthesis. The Mn4Ca cluster exists in mixed(IV/III) valence states, and facilitate water splitting using oxidation power increased through photochemical reactions. In 2011, the detailed molecular structure of the Mn4Ca cluster was first elucidated from the crystalline structure of PSII derived from thermophilic cyanobacteria at 1.9 A resolution (Y. Umena, et al. Nature, 2011). However, various debates are ongoing regarding the valence of each of the four Mn atoms, based on the local structure using X-ray crystal structure alalysis or theoretical calculations, and spectroscopy such as X-ray absorption and electron spin resonance. Here I will introduce our latest study to elucidate each electronic state of the four Mn atoms based on the crystalline structure analysis using X-rays across Mn K-edge resonance. It is expected that simultaneously ascertaining the local structure and the electron state of the active metal center will enable us to further elucidate the physicochemical properties of the enzymatic reactions.