  The crystal structure and electrical resistivity of YBa$_2$Cu$_4$O$_8$ (Y124)
were studied under high pressure up to 18 GPa using diamond-anvil cells,
respectively, in order to clarify its conduction mechanism. Y124 causes the
first-order phase-transition into the orthorhombic Immm at pressure around 11
GPa. The high-pressure phase (HPP) also shows the superconductivity, while the
manner of temperature dependence of electrical resistance and the pressure
dependence of transition temperature, Tc, drastically change above 11 GPa. The
CuO$_2$ plane persists in HPP but the CuO double chains collapse with the phase
transition and transform into three-dimensional Cu-O network, resulting in the
renewal of conduction system.
