  We examine the electronic states of Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ by the
ionic and cluster model approach. It is found that self-doped holes are likely
to stay on the chain at $x=0$ and the Ca substitution drives the holes to move
to the ladder. This feature is caused by the change of the positions of (Sr,Ca)
layers, which enhances the electrostatic potentials in the chain. The optical
conductivity is calculated applying the exact diagonalization method to small
Cu-O clusters. It is shown that the excitations below 2 eV are governed by the
ladder, giving rise to the spectral weight transfer from the charge-transfer
excitation around 2 eV to low energy Drude excitation, while the contribution
from the chain mainly emerges in higher energy region showing large weight
around 3 eV. These provide consistent explanation to recent experiment.
