  Photo-induced spin-state change in itinerant correlated-electron system is
studied. The model Hamiltonians before and after photon-pumping are derived
from the two-orbital Hubbard model with crystalline field splitting. A photon
introduced in the low-spin band insulator induces a bound state of the
high-spin state and a photoexcited hole. This bound state brings a
characteristic peak in the optical absorption spectra in the photo-excited
state. The resent results well explain the recent experimental results of the
ultrafast optical spectroscopy in perovskite cobaltites.
