When did round disk galaxies form?
  When and how galaxy morphology such as disk and bulge seen in the present-day
universe emerged is still not clear. In the universe at $z\gtrsim 2$, galaxies
with various morphology are seen, and star-forming galaxies at $z\sim2$ show an
intrinsic shape of bar-like structure. Then, when did round disk structure
form? Here we take a simple and straightforward approach to see the epoch when
a round disk galaxy population emerged by constraining the intrinsic shape
statistically based on apparent axial ratio distribution of galaxies. We
derived the distributions of the apparent axial ratios in the rest-frame
optical light ($\sim 5000$ \AA) of star-forming main sequence galaxies at
$2.5>z>1.4$, $1.4>z>0.85$, and $0.85>z>0.5$, and found that the apparent axial
ratios of them show peaky distributions at $z\gtrsim0.85$, while a rather flat
distribution at the lower redshift. By using a tri-axial model ($A>B>C$) for
the intrinsic shape, we found the best-fit models give the peaks of the $B/A$
distribution of $0.81\pm0.04$, $0.84\pm0.04$, and $0.92\pm0.05$ at $2.5>z>1.4$,
$1.4>z>0.85$, and $0.85>z>0.5$, respectively. The last value is close to the
local value of 0.95. Thickness ($C/A$) is $\sim0.25$ at all the redshifts and
is close to the local value (0.21). The results indicate the shape of the
star-forming galaxies in the main sequence changes gradually, and the round
disk is established at around $z\sim0.9$. Establishment of the round disk may
be due to a cease of violent interaction of galaxies or a growth of a bulge
and/or a super-massive black hole resides at the center of a galaxy which
dissolves the bar structure.
