  The electronic structures of Sr_{14-x}Ca_{x}Cu24O41 are calculated within the
local density approximation. Around the Fermi energy there exist
quasi-one-dimensional bands originated from the ladder and chain layers. The
nearest-neighbor inter-ladder hoppings are estimated to be 5--20% of the
intra-ladder ones. Possible effects of Ca substitution on electronic structures
and charge distribution are also discussed.
