Through analyses of the density-matrix equations of motion for electrons in metals, we show that the energy band gap is enhanced when many electrons are excited from core to valence bands, indicating a tendency opposite to the band-gap reduction observed in the valence-electron excitation of semiconductors; it is a consequence of inhomogeneous excitation from highly localized core to delocalized valence bands. Such a band-gap enhancement suggests a blue shift of the photoabsorption edge, leading to a possibility of demonstrating saturable absorber switching by means of intense EUV or X-ray free-electron-laser pulses.