Taniguchi has shown in [9] that the same uniqueness theorem isvalid for some first order symmetric hyperbolic systems in the case whenIn this paper, we intend to extend his result to the case when n>iand to more general hyperbolic systems, which, in particular, enablesus to discuss the above uniqueness of solutions of the Dirac equation.The authors wish to express their sincere thanks to Prof. R. linofor his encouragement and kind suggestions.After this paper was submitted, the authors were informed of theHormander's work entitled Lower bounds at infinity for solutions ofdifferential equations with constant coefficients (Israel J. Math., 16,103-116 (1973)) by Prof. M. Murata. His results contain a part ofTheorem 2.1 in this paper. The authors thank to Prof. M. Murata forhis kind announcement.