We report on a novel phase transition at T = 0.9 K in the Ce-based filled-skutterudite compound CeOs4Sb12 via measurements of the nuclear-spin lattice relaxation rate 1/T1 and nuclear quadrupole resonance (NQR) spectrum of Sb nuclei. The temperature (T ) dependence of 1/T1 behaves as if approaching closely an antiferromagnetic (AFM) quantum critical point (QCP), following the relation 1/T1 ∝ T /(T − TN) 1/2 with TN = 0.06 K in the range of T = 1.3 − 25 K. The onset of either the spin-density-wave (SDW) or charge-density-wave (CDW) order at T0 = 0.9 K, that is, of the first order, is evidenced by a broadening of the NQR spectrum and a marked reduction in 1/T1 just below T0. The f -electron-derived correlated band realized in CeOs4Sb12 is demonstrated to give rise to the novel phase transition on the verge of AFM QCP.KEYWORDS: filled skutterudite, Kondo semiconductor, quantum critical point, SDW/CDW, NQR, CeOs4Sb12Filled-skutterudite compounds ReT 4 Pn 12 (Re = rare earth; T = Fe, Ru or Os; Pn = P, As or Sb) have attracted much attention because of their rich physical phenomena that are not yet fully understood. For instance, PrFe 4 P 12 shows a heavy fermion (HF)-like behavior with a large mass of m * ∼ 70 m e under a magnetic field (H) 1 and undergoes an anomalous transition at a temperature T = 6.4 K at H = 0, indicative of a quadrupolar ordering. [2][3][4] Note that the large C/T value for this compound is due not only to low-energy degrees of freedom such as either magnetic or quadrupolar fluctuations, but also to the Schottky anomaly originating from some low-lying CEF splitting. PrOs 4 Sb 12 is the first Pr-based HF superconductor to reveal the large jump in the specific heat at T c = 1.85 K, the slope of the upper critical field near T c , and the electronic specific heat coefficient γ ∼ 350 − 500 mJ/K 2 mol in the normal state. 5 The CEF-energy scheme of PrOs 4 Sb 12 has recently been determined to be the singlet Γ 1 ground state accompanying the very low lying excited state Γ (2) 4 , 6, 7 and it was suggested that the HF-like behavior exhibited by PrOs 4 Sb 12 may be relevant to a very close energy level between Γ 1 and Γ 4 . Thus, in the Pr-based compound, the 4f 2 -derived CEF effect plays an important role in their rich physical phenomena.In contrast, most Ce-based filled-skutterudite compounds show semiconducting behavior, on the basis of which they are called hybridization-gap semiconductors. A series of CeT 4 P 12 (T = Fe, Ru or Os) compounds have a hybridization gap of 400∼1500 K, and *