  We consider the femto-lensing due to a cosmic string. If a cosmic string with
the deficit angle $\Delta\sim 100$ [femto-arcsec] $\sim10^{-18}$ [rad] exists
around the line of sight to a gamma-ray burst, we may observe characteristic
interference patterns caused by gravitational lensing in the energy spectrum of
the gamma-ray burst. This "femto-lensing" event was first proposed as a tool to
probe small mass primordial black holes. In this paper, we propose use of the
femto-lensing to probe cosmic strings with extremely small tension.
Observability conditions and the event rate are discussed. Differences between
the cases of a point mass and a cosmic string are presented.
