We have carried out nuclear quadrupole resonance (NQR) measurements in the filled skutterudite compounds Pr(Os1-x,Ru,)(4)Sb-12 (x = 0. 1, 0.2), in order to gain insights into the symmetry of the superconducting gap function. Upon replacing Os with Ru, the spin-lattice relaxation rate 1/T-1 becomes proportional to temperature (T) at low T far below T-c, and the magnitude of (1/T1T1)(low-T) increases with increasing Ru content. These results indicate that a finite density of states is induced at the Fermi level by the impurity, and thus suggest that there exist nodes in the gap function of PrOs4Sb12.