  We study the effect of a probe string to black hole complexity according to
the CA (Complexity equals Action) conjecture. Our system contains a particle
moving on the boundary of black hole spacetime. In the dual description this
corresponds to the insertion of a fundamental string on the bulk spacetime. The
total action consists of the Einstein-Hilbert term and the Nambu-Goto term. The
effect of this string is expressed by the Nambu-Goto term. Focusing on the
Nambu-Goto term, we analyse the time development of this system. Our results
show some interesting properties of complexity. This gives a useful hint for
defining complexity in quantum field theories.
