  A localization length of a free-spin soliton from a non-magnetic impurity is
deduced in a general double-spin-chain model ($J_0-J_1-J_2-J_3$ model). We have
solved a variational problem which employs the nearest-neighbor singlet-dimer
basis. The wave function of a soliton is expressed by the Airy function, and
the localization length $(\xi)$ is found to obey a power law of the
dimerization $(J_2-J_3)$ with an exponent -1/3; $\xi\sim (J_2-J_3)^{-1/3}$.
This explains why NaV_2O_5 does not show the antiferromagnetic order, while
CuGeO_3 does by impurity doping. When the gap exists by the bond-dimerization,
a soliton is localized and no order is expected. Contrary, there is a
possibility of the order when the gap is mainly due to frustration.
