  It is shown by the Wilson's numerical renormalization-group approach that
Non-Fermi liquid (NFL) behaviors arise even for U-impurity with an
$f^2$-singlet ground state, and all the puzzling behaviors of
R$_{1-x}$U$_x$Ru$_2$Si$_2$ (R=Th, Y and La, $x\le 0.07$) can be explained on
its basis. There exists an unstable NFL fixed point due to the balance between
Kondo singlet and Crystalline-Electric-Field singlet ground states. For wide
region in parameter space the system shows the NFL behavior as a transient
phenomenon in temperature range accessible by experiment. Results for
temperature dependence of the resistivity, the magnetic susceptibility and the
Sommerfeld coefficient with and without magnetic field reproduce the behaviors
observed in Th$_{1-x}$U$_x$Ru$_2$Si$_2$. Less anomalous properties of
R$_{1-x}$U$_x$Ru$_2$Si$_2$ (R=Y and La) can be understood along the same
scenario provided that those compounds have different sets of parameters.
