  The accurate understanding of flux dynamics is essential for the design and
operation of superconducting circuits. The time evolution of flux-density
distribution in an NbN strip by the transport current was observed using
high-speed magneto-optical microscopy. It was determined that even for the
dynamic penetration and exclusion of vortices under the transport current, the
surface barrier is essential. This feature is important for the correct
understanding of the complex behavior of state-of-the-art superconducting
devices.
