  We investigate the weak lensing effects of line-of-sight structures on
quadruple images in quasar-galaxy strong lens systems based on N-body and
ray-tracing simulations that can resolve halos with a mass of 10^5 solar mass.
The intervening halos and voids disturb the magnification ratios of lensed
images as well as their relative positions due to lensing. The magnification
ratios typically change by O(10%) when the shifts of relative angular positions
of lensed images are constrained to <0.004 arcsec. The constrained amplitudes
of projected density perturbations due to line-of-sight structures are O(10^8)
solar mass per arcsec^2. These results are consistent with our new analytical
estimate based on the two-point correlation of density fluctuations. The
observed mid-infrared (MIR) flux ratios for 6 quasar-galaxy lens systems with
quadruple images agree well with the numerically estimated values without
taking into account of subhalos residing in the lensing galaxies. We find that
the constrained mean amplitudes of projected density perturbations in the
line-of-sight are negative, which suggests that the fluxes of lensed images are
perturbed mainly by minivoids and minihalos in underdense regions. We derive a
new fitting formula for estimating the probability distribution function of
magnification perturbation. We also find that the mean amplitude of
magnification perturbation roughly equals the standard deviation regardless of
the model parameters.
