  We report on the discovery of a star-forming galaxy at z=6.33 in the Subaru
Deep Field. This object is selected as a candidate of an i'-dropout,
high-redshift galaxy around z=6 because of its red i'-z' color in our deep
optical imaging survey in the Subaru Deep Field. Our follow up optical
spectroscopy reveals that this object is a strong Ly-alpha emitter with only
very faint ultraviolet continuum. The rest-frame equivalent width of the
detected Ly-alpha emission is as much as 130 A. Thus the light detected in our
z' image is largely attributed to the Ly-alpha emission, i.e., ~40% of the
z'-band flux is the strong Ly-alpha emission, giving a very red i'-z' color.
This is consistent with the photometric property of this object because the
narrow-band data obtained with the NB921 filter shows a significant depression,
z'-NB921 = -0.54 mag. By using the photometric data, we show that some other
objects among the 48 i'-dropout high-redshift galaxy candidates found in the
Subaru Deep Field also show a significant NB921 depression. We briefly discuss
the nature of these NB921-depressed objects.
