  We present exact solutions of the massless Klein-Gordon equation in a
spacetime in which an infinite straight cosmic string resides. The first
solution represents a plane wave entering perpendicular to the string
direction. We also present and analyze a solution with a static point-like
source. In the short wavelength limit these solutions approach the results
obtained by using the geometrical optics approximation: magnification occurs if
the observer lies in front of the string within a strip of angular width $8\pi
G\mu$, where $\mu$ is the string tension. We find that when the distance from
the observer to the string is less than $ 10^{-3} {(G \mu)}^{-2}\lambda \sim
150 {\rm Mpc} (\lambda/{\rm AU}) (G\mu/10^{-8})^{-2}$, where $\lambda$ is the
wave length, the magnification is significantly reduced compared with the
estimate based on the geometrical optics due to the diffraction effect. For
gravitational waves from neutron star(NS)-NS mergers the several lensing events
per year may be detected by DECIGO/BBO.
