  We investigate the weak lensing effect by line-of-sight structures with a
surface mass density of <~10^8 solar mass/arcsec^2 in QSO-galaxy quadruple lens
systems. Using high-resolution N-body simulations in warm dark matter (WDM)
models and observed four quadruple lenses that show anomalies in the flux
ratios, we obtain constraints on the mass of thermal WDM, m_WDM>= 1.3keV(95%CL)
assuming that the density of the primary lens is described by a singular
isothermal ellipsoid (SIE). The obtained constraint is consistent with those
from Lyman-$\alpha$ forests and the number counts of high-redshift galaxies at
z>4. Our results show that WDM with a free-streaming comoving wavenumber k_{fs}
<= 27 h/Mpc is disfavored as the major component of cosmological density at
redshifts 0.5 <~ z <~ 4 provided that the SIE models describe the gravitational
potentials of the primary lenses correctly.
