  Recently it has been claimed that the warm dark matter (WDM) model cannot at
the same time reproduce the observed Lyman-{\alpha} forests in distant quasar
spectra and solve the small-scale issues in the cold dark matter (CDM) model.
As an alternative candidate, it was shown that the mixed dark matter (MDM)
model that consists of WDM and CDM can satisfy the constraint from
Lyman-{\alpha} forests and account for the "missing satellite problem" as well
as the reported 3.5 keV anomalous X-ray line. We investigate observational
constraints on the MDM model using strong gravitational lenses. We first
develop a fitting formula for the nonlinear power spectra in the MDM model by
performing N-body simulations and estimate the expected perturbations caused by
line-of-sight structures in four quadruply lensed quasars that show anomaly in
the flux ratios. Our analysis indicates that the MDM model is compatible with
the observed anomaly if the mass fraction of the warm component is smaller than
0.47 at the 95% confidence level. The MDM explanation to the anomalous X-ray
line and the small-scale issues is still viable even after this constraint is
taken into account.
