  We have performed the photoemission and inverse photoemission experiments to
elucidate the origin of Mott insulating states in A-site ordered perovskite
CaCu$_3$Ti$_4$O$_{12}$ (CCTO). Experimental results have revealed that Cu
3$d$-O 2$p$ hybridized bands, which are located around the Fermi level in the
prediction of the local-density approximation (LDA) band calculations, are
actually separated into the upper Hubbard band at $\sim$ 1.5 eV and the lower
Hubbard band at $\sim$ $-$1.7 eV with a band gap of $\sim$ 1.5-1.8 eV. We also
observed that Cu 3$d$ peak at $\sim$ $-$3.8 eV and Ti 3$d$ peak at $\sim$ 3.8
eV are further away from each other than as indicated in the LDA calculations.
In addition, it is found that the multiplet strucutre around $-$9 eV includes a
considerable number of O 2$p$ states. These observations indicate that the Cu
3$d$ and Ti 3$d$ electrons hybridized with the O 2$p$ states are strongly
correlated, which originates in the Mott-insulating states of CCTO.
