¹¹B NMR measurements have been performed on rare earth ternary borides, YRuB₂, LuRuB₂ and (Tm_0.4Y_0.6)RuB₂ to investigate their electronic states. From the measurements of temperature dependence of spin-lattice relaxation rate 1/T₁, YRuB₂ and LuRuB₂ are found to be Bardeen-Cooper-Schrieffer (BCS) superconductors with the energy gap 2Δ(0)=3.52 k_BT_c and the superconducting transition temperatures T_c=7.6 K (YRuB2) and 10.2 K (LuRuB₂). A very broad and temperature dependent spectrum is observed in (Tm_0.4Y_0.6)RuB₂. The
magnitude of 1/T₁ T is considerably larger in (Tm_0.4Y_0.6)RuB₂ than in YRuB₂ and LuRuB₂. These facts suggest the contribution of 4¦ electrons to the hyperfine field at ¹¹B nuclei in (Tm_0.4Y_0.6)RuB₂.