DNA-binding proteins recognize DNA sequences with at least two different binding modes: specific and nonspecific. Experimental structures of such complexes provide us a static view of the bindings. However, it is difficult to reveal further mechanisms of their target-site search and recognition only from static information because the transition, process between the bound and unbound states is not clarified by static information. What is the difference between specific and nonspecific bindings? Here we performed adaptive biasing force molecular,dynamics simulations with the specific and nonspecific structures of DNA-Lac repressor complexes to investigate the dissociation process. The resultant free-energy profiles showed that the specific complex has a sharp, deep well consistent with tight binding, whereas the nonspecific complex has a, broad, shallow well consistent with loose binding. The difference in the well depth, similar to 5 kcal/mol, was in fair agreement with the, experimentally obtained value and was found to mainly come from the protein conformational difference, particularly in the C-terminal tail. Also, the free-energy profiles were,found to be correlated with changes in the number of protein-DNA contacts and that of surface water molecules. The derived protein spatial distributions around the DNA indicate that any large dissociation occurs rarely, regardless of the specific and nonspecific sites. Comparison of the free-energy barrier for sliding [similar to 8.7 kcal/mol; Furini et al. J. Phys. Chem. B 2010, 114, 2238] and that for dissociation (at least similar to 16 kcal/mol) calculated in this study suggests that sliding is much preferred to dissociation.