A rapid temperature‐jump (T‐J) device was designed for the kinetic measurements of small‐angle x‐ray scattering from solutions of biological macromolecular systems. The dead time of the apparatus is reduced to a few hundred milliseconds. The structural change of biological macromolecules in solutions can be monitored in the time scale of a few hundred milliseconds by the synchrotron small‐angle x‐ray scattering with a combination of a CAMAC data‐acquisition system and the present T‐J apparatus.