  In the quasi-one-dimensional superconductor,
Pr$_2$Ba$_4$Cu$_7$O$_{15-\delta}$, we found that the lattice parameter, $a$
increased against pressure, starting at 2.0 GPa at approximately 300 K. This
result suggests that the two-dimensional inter-conducting-chain interaction
along the $a$ axis is most enhanced at approximately 2.0 GPa. To discuss the
dimensionality, we also measured the resistance under magnetic fields up to 14
T, because magneto-resistance is induced when a system changes from
one-dimensional to two-dimensional.
