A block copolymer poly(styrene-b-tert-butylstyrene) in the frozen "disordered" state has been prepared by fast solution casting. The time changes in the scattered intensity induced by a temperature jump from the frozen disordered state at room temperature to above the glass transition temperature were measured by using time-resolved synchrotron small-angle X-ray scattering. It was found that the time changes in the scattered intensity could be approximated by the Cahn linearized theory. The wavenumber q-dependence of the Onsager coefficient, Lambda(q), derived from the growth rate, R(q), was discussed.