A structural phase transition has been found using electron diffraction tec
hnique in PrRu4P12 accompanied by a metal-insulator (M-I) transition (TMI =
60 K). Weak superlattice spots appeared at (H, K, L) (H + K + L = odd) pos
ition at a temperature of T = 12 K and 40 K. Above T = 70 K, the spots comp
letely vanished. The space group of the low-temperature phase is probably P
m (3) over bar. This is the first observation of a symmetry other than Im (
3) over bar in skutterudite compounds.