In this manuscript we consider the conjugate notion focused from consumer theory as an interesting tool. According to us, conjugate notion remained undeveloped in economic theory because Fenchel's conjugate notion was introduced exclusively for proper convex lower semi continuous functions and convexity assumption is not natural in economic theory. Nevertheless, we introduce necessary and sufficient optimality conditions for consumer problem. Also, we consider a particular version of Fenchel–Moreau conjugate notion, for lower semi continuous functions recently introduced in the literature as a generalization of Fenchel's conjugate. Finally, we adapt it to closed functions in order to define a convex dual problem for consumer problem.