[en] Specific heat measurements on a series of ternary compound RCu₂Si₂ (R: La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) single crystals have been performed by the quasi-adiabatic heat pulse method in the wide temperature range from 0.1 to 80 K. Almost all the compounds show antiferromagnetic orderings at low temperatures. We found the first-order like very sharp specific heat peaks in RCu₂Si₂ with R=Gd, Tb, Dy, Ho, and Er. Especially, the antiferromagnetic ordering in DyCu₂Si₂ is found to be a first-order transition. From the analyses of the temperature dependence of magnetic entropy, we confirmed that the total splitting energy of the crystalline electric field (CEF) scheme of R³⁺ ion in RCu₂Si₂ is about 100 K in all the compounds studied here, except for YbCu₂Si₂. In addition, we observed the nuclear specific heat at low temperatures and estimated the magnitude of internal effective magnetic field H_eff at the nucleus. (author)