  From deep H-alpha imaging data of Suprime-Cam/Subaru, we discovered nine
extended ionized gas clouds (EIG) around galaxies in Abell 851 cluster (A851)
at z=0.4. We surveyed 30 x 25 arcmin region, and the EIGs were found only near
the cluster center (<2.3 arcmin ~ 750 kpc). The parent galaxies of the EIGs are
star-forming or post-starburst galaxies, all of which are spectroscopically
confirmed members of the cluster. Four out of the nine parent galaxies show
distortion of stellar distribution in the disk, which can be a sign of recent
interaction, and the interaction may have made EIGs. On the other hand, six
parent galaxies (one overlaps those exhibiting distortion) show H-alpha
emission without stars, which implies a ram pressure stripping.The spe ctrum of
the brightest parent galaxy shows a post-starburst signature, and resembles the
H-alpha stripped galaxies found in the Coma cluster. Meanwhile, two brightest
parent galaxies in A851 are more massive than the EIG parent galaxies in the
Coma cluster. This is consistent with "downsizing" of star-forming galaxies,
though it is still in a statistical fluctuation. We also analyzed Suprime-Cam
data of another z=0.39 cluster, CL0024+17, but found no EIGs. The key
difference between A851 and CL0024+17 would be the existence of a subcluster
colliding with the main body of A851, in which six or seven out of the nine
parent galaxies in A851 exist, and the fraction of EIGs in the subcluster is
significantly higher than the main subcluster of A851 and CL0024+17.
