The oxidation state of Ce in zircons of the Cretaceous Habu and Yamanaka granodiorite from Southwest Japan was measured by Ce LIII-edge X-ray absorption near-edge structure (XANES). The results confirmed that tetravalent Ce is incorporated in zircon, as estimated from the positive Ce anomaly in the REE pattern of zircons. However, the CeIV percentage in Yamanaka zircon is much smaller than expected from the degree of its Ce anomaly. The γ-irradiation experiments suggest that the decrease of CeIV can be well explained by the reductive reaction of CeIV to CeIII by the radiation effect due to U and Th in the zircon. This suggests that it is possible to date zircon by the amount of Ce reduced after sample formation. As shown in this, it is possible to deduce geochemical information by comparing the CeIV percentage at present given by XANES and the initial CeIV percentage estimated from Ce anomaly.