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We report X-ray, optical, and near-infrared monitoring of the new X-raytransient MAXI J1820$+$070 discovered with MAXI on 2018 March 11. Its X-rayintensity reached $\sim 2$ Crab in 2--20 keV at the end of March, and thengradually decreased until the middle of June. In this period, the X-rayspectrum was described by Comptonization of the disk emission, with a photonindex of $\sim$1.5 and an electron temperature of $\sim$50 keV, which isconsistent with a black hole X-ray binary in the low/hard state. The electrontemperature and the photon index were slightly decreased and increased withincreasing flux, respectively. The source showed significant X-ray fluxvariation on timescales of seconds. This short-term variation was found to beassociated with changes in the spectral shape, and the photon index becameslightly harder at higher fluxes. This suggests that the variation was producedby a change in the properties of the hot electron cloud responsible for thestrong Comptonization. Modeling a multi-wavelength SED around the X-ray fluxpeak at the end of March, covering the near-infrared to X-ray bands, we foundthat the optical and near-infrared fluxes were likely contributed substantiallyby the jet emission. Before this outburst, the source was never detected in theX-ray band with MAXI (with a 3$\sigma$ upper limit of $\sim$0.2 mCrab in 4--10keV, obtained from the 7-year data in 2009--2016), whereas weak optical andinfrared activity was found at their flux levels $\sim$3 orders of magnitudelower than the peak fluxes in the outburst.

X-ray, Optical, and Near-infrared Monitoring of the New X-ray Transient MAXI J1820+070 in the Low/hard State