  The hot sub-Saturn-mass exoplanet HAT-P-12b is an ideal target for
transmission spectroscopy because of its inflated radius. We observed one
transit of the planet with the multi-object double spectrograph (MODS) on the
Large Binocular Telescope (LBT) with the binocular mode and obtained an
atmosphere transmission spectrum with a wavelength coverage of $\sim$ 0.4 --
0.9 $\mathrm{\mu}$m. The spectrum is relatively flat and does not show any
significant sodium or potassium absorption features. Our result is consistent
with the revised Hubble Space Telescope (HST) transmission spectrum of a
previous work, except that the HST result indicates a tentative detection of
potassium. The potassium discrepancy could be the result of statistical
fluctuation of the HST dataset. We fit the planetary transmission spectrum with
an extensive grid of cloudy models and confirm the presence of high-altitude
clouds in the planetary atmosphere. The fit was performed on the combined LBT
and HST spectrum, which has an overall wavelength range of 0.4 -- 1.6
$\mathrm{\mu}$m. The LBT/MODS spectrograph has unique advantages in
transmission spectroscopy observations because it can cover a wide wavelength
range with a single exposure and acquire two sets of independent spectra
simultaneously.
