In this paper, first, we establish the solvability of (n × n)-systems of hyperbolic type with inhomogeneous mixed Neumann conditions involving operator of infinite order. Using theorems of J. L. Lions, quadratic boundary control problem for this system is considered. The necessary and sufficient conditions for the optimality of the control is obtained and the set of inequalities that characterize these conditions is formulated. Finally, by applying the DubovitskiiMilyutin theorem of W. Kotarski, necessary and sufficient conditions of optimality are derived for the same problem, where the performance index is more general than the quadratic one and has an integral form.