  The results from Suzaku observations of the galaxy cluster Abell2256 are
presented. This cluster is a prototypical and well-studied merging system,
exhibiting substructures both in the X-ray surface brightness and in the radial
velocity distribution of member galaxies. There are main and sub components
separating by 3'.5 in the sky and by about 2000 km s$^{-1}$ in radial velocity
peaks of member galaxies. In order to measure Doppler shifts of iron K-shell
lines from the two gas components by the Suzaku XIS, the energy scale of the
instrument was evaluated carefully and found to be calibrated well. A
significant shift of the radial velocity of the sub component gas with respect
to that of the main cluster was detected. All three XIS sensors show the shift
independently and consistently among the three. The difference is found to be
1500 $\pm 300$ (statistical) $\pm 300$ (systematic) km s$^{-1}$. The X-ray
determined absolute redshifts of and hence the difference between the main and
sub components are consistent with those of member galaxies in optical. The
observation indicates robustly that the X-ray emitting gas is moving together
with galaxies as a substructure within the cluster. These results along with
other X-ray observations of gas bulk motions in merging clusters are discussed.
