  To set useful limits on the abundance of small-scale dark matter halos
(subhalos) in a galaxy scale, we have carried out mid-infrared imaging and
integral-field spectroscopy for a sample of quadruple lens systems showing
anomalous flux ratios. These observations using Subaru have been successful for
distinguishing millilensing by subhalos from microlensing by stars. Current
status for our lensing analysis of dark matter substructure is reported.
