  The $\Lambda$CDM cosmological model successfully reproduces many aspects of
the galaxy and structure formation of the universe. However, the growth of
large-scale structures (LSSs) in the early universe is not well tested yet with
observational data. Here, we have utilized wide and deep optical--near-infrared
data in order to search for distant galaxy clusters and superclusters
($0.8<z<1.2$). From the spectroscopic observation with the Inamori Magellan
Areal Camera and Spectrograph (IMACS) on the Magellan telescope, three massive
clusters at $z\sim$0.91 are confirmed in the SSA22 field. Interestingly, all of
them have similar redshifts within $\Delta z\sim$0.01 with velocity dispersions
ranging from 470 to 1300 km s$^{-1}$. Moreover, as the maximum separation is
$\sim$15 Mpc, they compose a supercluster at $z\sim$0.91, meaning that this is
one of the most massive superclusters at this redshift to date. The galaxy
density map implies that the confirmed clusters are embedded in a larger
structure stretching over $\sim$100 Mpc. $\Lambda$CDM models predict about one
supercluster like this in our surveyed volume, consistent with our finding so
far. However, there are more supercluster candidates in this field, suggesting
that additional studies are required to determine if the $\Lambda$CDM
cosmological model can successfully reproduce the LSSs at high redshift.
