  X-ray and electron diffraction measurements on the metal-insulator (M-I)
transition compound PrRu$_4$P$_{12}$ have revealed the emergence of a periodic
ordering of charge density around the Pr atoms. It is found that the ordering
is associated with the onset of a low temperature insulator phase. These
conclusions are supported by the facts that the space group of the crystal
structure transforms from Im$\bar{3}$ to Pm$\bar{3}$ below the M-I transition
temperature and also that the temperature dependence of the superlattice peaks
in the insulator phase follows the squared BCS function. The M-I transition
could be originated from the perfect nesting of the Fermi surface and/or the
instability of the $f$ electrons.
