  We present mid-infrared imaging at 11.7 $\mu$m for the quadruple lens
systems, MG0414+0534 and Q2237+030, using the cooled mid-infrared camera and
spectrometer (COMICS) attached on the Subaru telescope. MG0414+0534 is
characterized by a bright pair of lensed images (A1, A2) and their optical flux
ratio A2/A1 deviates significantly from the prediction of a smooth lens model.
Q2237+030 is comprised of four lensed images, which are significantly affected
by microlensing in a foreground lensing galaxy. Our mid-infrared observations
of these lensed images have revealed that the mid-infrared flux ratio for A2/A1
of MG0414+0534 is nearly unity (0.90 +- 0.04). We find that this flux ratio is
systematically small, at 4 to 5 sigma level, compared with the prediction of a
best smooth lens model (1.09) represented by a singular isothermal ellipsoid
and external shear, thereby suggesting the presence of substructures to explain
our observational result. In contrast, for Q2237+030, our high signal-to-noise
observation indicates that the mid-infrared flux ratios between all the four
images of Q2237+030 are virtually consistent with the prediction of a smooth
lens model. Based on the size estimate of the dust torus surrounding the nuclei
of these QSOs, we set limits on the mass of a substructure in these lens
systems, which can cause anomalies in the flux ratios. For MG0414+0534, since
the required mass of a substructure inside its Einstein radius is > 360 M_sun,
millilensing by a CDM substructure is most likely. If it is modeled as a
singular isothermal sphere, the mass inside radius of 100 pc is given as >
1.0x10^5 M_sun. For Q2237+030, there is no significant evidence of
millilensing, so the reported anomalous flux ratios in shorter wavelengths are
entirely caused by microlensing by stars (abridged).
