  We report X-ray, optical, and near-infrared monitoring of the new X-ray
transient MAXI J1820$+$070 discovered with MAXI on 2018 March 11. Its X-ray
intensity reached $\sim 2$ Crab in 2--20 keV at the end of March, and then
gradually decreased until the middle of June. In this period, the X-ray
spectrum was described by Comptonization of the disk emission, with a photon
index of $\sim$1.5 and an electron temperature of $\sim$50 keV, which is
consistent with a black hole X-ray binary in the low/hard state. The electron
temperature and the photon index were slightly decreased and increased with
increasing flux, respectively. The source showed significant X-ray flux
variation on timescales of seconds. This short-term variation was found to be
associated with changes in the spectral shape, and the photon index became
slightly harder at higher fluxes. This suggests that the variation was produced
by a change in the properties of the hot electron cloud responsible for the
strong Comptonization. Modeling a multi-wavelength SED around the X-ray flux
peak at the end of March, covering the near-infrared to X-ray bands, we found
that the optical and near-infrared fluxes were likely contributed substantially
by the jet emission. Before this outburst, the source was never detected in the
X-ray band with MAXI (with a 3$\sigma$ upper limit of $\sim$0.2 mCrab in 4--10
keV, obtained from the 7-year data in 2009--2016), whereas weak optical and
infrared activity was found at their flux levels $\sim$3 orders of magnitude
lower than the peak fluxes in the outburst.
