We derive an analytic expression of the effective potential at finite temperature (T) and chemical potential (mu) in the strong-coupling lattice QCD for color SU(3) including next-to-next-to-leading order (NNLO) effects in the strong coupling expansion. NNLO effective action terms are systematically evaluated in the leading order of the large dimensional (1/d) expansion, and are found to come from some types of connected two plaquette configurations. We apply the extended Hubbard-Stratonovich

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... ansformation and a gluonic dressed fermion technique to the effective action, and obtain the effective potential as a function of T, mu, and two order parameters; chiral condensate and a vector potential field. The next-to-leading order (NLO) and NNLO effects result in modifications of the wave function renormalization factor, quark mass and chemical potential. We find that T_c,mu=0 and mu_c,T=0 are similar to the NLO results, whereas the position of the critical point is sensitive to NNLO corrections.