195Pt-, ¹³⁹La- and ¹¹B-NMR have been investigated in the quaternary superconductors RT₂B₂C (R=Y,La,Th and T=Ni,Pd,Pt). The electric quadrupole frequency of ¹³⁹La changes almost by one order of magnitude from T=Ni to Pt, indicating that the hybridization of La 5d and d-electrons of the transition element is important and depends largely on the transition element. T₁T of ¹⁹⁵Pt obeys the Korringa relation in the normal state, but ¹¹B-1/T₁T and Knight shift are enhanced with decreasing temperature in some borides. The temperature dependence of T1 of ¹⁹⁵Pt in ThPt2B2C below Tc strongly suggests that the conventional BCS superconductivity is realized in the quaternary borocarbide.