Ultrafast photoexcitation of the spin-polarized Dirac surface state in the topological insulator Sb₂Te₃ was investigated by using time- and angle-resolved two-photon photoemission spectroscopy combined with tunable mid-infrared pump pulses. It was revealed that mid-infrared pump permits a direct excitation between the occupied and unoccupied part of the surface Dirac-cone. Moreover, the direct optical coupling induces asymmetric transient populations in the surface states at ±k_||, which reflects a macroscopic photoexcited electric surface current. By observing the decay of the asymmetric population and its temperature dependence, the ultrafast dynamics of the surface current was visualized in the momentum space.