Magnetic tunnel junctions (MTJs) consisting of two ferromagnetic electrode layers separated by a crystalline MgO(001) tunneling barrier exhibit a giant tunneling magnetoresistance (TMR) effect at room temperature. The effect is a key for the development of next-generation spintronic devices such as magnetoresistive random access memory (MRAM) and ultrahigh-density hard disk drives. First-principle theories predicted that no oxidation of magnetic atoms at the barrier/electrode interfaces is essential for the giant TMR effect. Wehave fabricated fully epitaxial MTJs such as Fe(001)/MgO(001)/Fe(001) using MBE technique and achieved huge magnetoresistance ratios above 400% at room temperature, the highest value reported so far. We investigated the interface structure using x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD), and confirmed that the interface Fe atoms are not oxidized and have an enhanced magnetic moment although they are neighboring the O atoms of MgO.