The dispersion relations with the ordinary threshold energies are proved in every order of perturbation expansion for π-π, π-K, π-N, π-Λ, π-Ξ K-K, N-N, Λ-Λ, N-Ξ, K-N, K-Λ forward scatterings. A counter-example is presented for N-Λ, N-Σ, Σ-Σ, Λ-Ξ, Σ-Ξ scatterings. Non-forward scatterings also are treated.