  We present improved constraints on the formation time scale of massive
cluster galaxies based on rest-frame optical spectra of galaxies in a forming
cluster located at z=2.16. The spectra are obtained with MOIRCS on the Subaru
telescope with an integration time of ~7 hours. We achieve accurate redshift
measurements by fitting SEDs using the spectra and broad-band photometry
simultaneously, allowing us to identify probable cluster members. Clusters at
low redshifts are dominated by quiescent galaxies, but we find that quiescent
galaxies and star forming galaxies co-exist in this z=2 system. Interestingly,
the quiescent galaxies form a weak red sequence in the process of forming. By
stacking the spectra of star forming galaxies, we observe strong emission lines
such as [OII] and [OIII] and we obtain a tentative hint of AGN activities in
these galaxies. On the other hand, the stacked spectrum of the quiescent
galaxies reveals a clear 4000A break with a possible CaII H+K absorption
feature and strong emission lines such as [OII] are absent in the spectrum,
confirming the quiescent nature of these galaxies. We then perform detailed
spectral analyses of the stacked spectrum, which suggest that these massive
quiescent galaxies formed at redshifts between 3 and 4 on a time scale of <~0.5
Gyr. This short formation time scale is not reproduced in recent numerical
simulations. We discuss possible mechanisms for how these galaxies form 10^11
Msun stellar mass on a short time scale and become red and quiescent by z=2.
