  The brightness temperature of the radio free-free emission at millimeter
range is an effective tool for characterizing the vertical structure of the
solar chromosphere. In this paper, we report on the first single-dish
observation of a sunspot at 85 and 115 GHz with sufficient spatial resolution
for resolving the sunspot umbra using the Nobeyama 45 m telescope. We used
radio attenuation material, i.e. a solar filter, to prevent the saturation of
the receivers. Considering the contamination from the plage by the side-lobes,
we found that the brightness temperature of the umbra should be lower than that
of the quiet region. This result is inconsistent with the preexisting
atmospheric models. We also found that the brightness temperature distribution
at millimeter range strongly corresponds to the ultraviolet (UV) continuum
emission at 1700 {\AA}, especially at the quiet region.
