Surface chemical states and oxidation resistivity of the 'ecologically friendly' semiconductor beta-FeSi2 have been investigated. Previously, we studied beta-FeSi2 thin films prepared by the ion-beam sputter deposition method (IBSD) on an Si(100) substrate. Through these studies, it was observed that the oxidation of the formed FeSi2 does not proceed so much even in the air atmosphere compared with elemental Si or Fe or other compound semiconductors. In the present study, the obtained films were analyzed by synchrotron-radiation X-ray photoelectron spectroscopy (SR-XPS) in order to obtain chemical states and depth profiles of films non-destructively. From the XPS spectra, the results indicated FeSi2 films have strong oxidation resistivity. The XPS spectra reveal that oxidation resistivity is caused by a very thin SiO2 layer covering the formed beta-FeSi2. The results of cross sectional observation by transmission electron microscope and surface morphological analysis by scanning electron microscope are consistent with the SR-XPS results.