The cerebral cortex is an indispensable region involved in higher cognitive functions in the mammalian brain and is particularly evolved in the primate brain. It has been demonstrated that the cortical areas are formed by both innate and activity-dependent mechanisms. However, it remains unknown what molecular changes induced cortical expansion and complexity during primate evolution. Active DNA methylation/demethylation is one of the epigenetic mechanisms that modify gene expression by methylating/demethylating promoter regions. Three growth arrest and DNA damage-inducible small nuclear proteins called Gadd45 alpha, beta, and gamma have been identified as regulators. To explore the involvement of epigenetic factors in primate cortical evolution, we performed gene expression analyses of these three demethylation genes in the developing common marmoset (Callithrix jacchus) brain. We found that the cortical expression levels of these genes were overall increased transiently and reduced during development. However, Gadd45b exhibited a high expression level in some areas of the adult brain, including the prefrontal cortex, temporal cortex, posterior parietal cortex, and insula, which are particularly expanded in greater primates and humans. Compared to the marmoset brain, there were no clear regional differences or developmentally enhanced expressions of these genes in the mouse brain. These results suggest the possibility that high-level expression of epigenetic regulators in the primate brain allow cortical expansion, complexity, and the ability to adapt to environmental factors.