We study the deconfinement phase transition in SU(3) with dynamical quarks at finite baryon-number density by considering a strong-coupling effective action. The effective action is studied by using the mean-field approximation. The quarks are incorporated in the Wilson formalism by adopting the hopping-parameter expansion. At fixed values of the hopping parameter kappa or chemical potential ..mu.., two degenerate minima of the mean-field free energy are observed and critical values of the parameters at phase-transition points are determined. The critical curve in the T-..mu.. plane is given.