  In present paper we study a boundary value problem for a mixed
parabolic-hyperbolic type equation in a rectangular domain and prove the
existence of unique solution of this problem. In theory of boundary value
problems for second order mixed type equations usually two conjugation
conditions are in use. In this case, for mixed type equations containing
hyperbolic equation in a rectangular domain for solvability of boundary value
problem appears certain condition. In this paper we give three conjugation
conditions. In this case mentioned condition not appears.
