Massive stellar cluster is a major site of galaxy nuclear starburst, and birth and
infancy of such massive stellar cluster are of great interest. Mm/sub-mm recombination
line suffers little interstellar extinction, and the characteristic makes it the most
effective means of unveiling dense ionized gas and infant massive stellar clusters which
are deeply embedded in natal interstellar matter.

ALMA observations toward the nearby starburst galaxy NGC 253 were carried out with high
spatial resolution (10 pc).  Hydrogen recombination line and archival millimeter wave
continuum data enable us to estimate ionized gas parameters, and they turn out to be
similar to or even higher than those of Compact and Ultracompact HII regions in the
Milky Way Galaxy.  The central part of NGC 253 is resolved into several compact sources
and each of them contains tens to a few hundred of O-type stars.  Our observation
results suggest the sources are dense ionized gas clumps embedded with aborning and/or
infant massive stellar clusters.