  We have obtained deep images in the BVRIJHKs bands of the field centered on
QSO 1508+5714 (z_{em} =4.28) with the Suprime camera, FOCAS and MOIRCS cameras
on Subaru telescope. We report here the detection of a B-dropout galaxy, which
is $3\secpoint 5$ north-west of the QSO sightline. A photometric redshift
analysis is presented to complement the color selection. Given the photometric
properties of this object ($M = -22.2$, making $ L\approx 3 L^{\ast}$, if
placed at its photometric redshift $z\sim 3.5$), as well as the
S$\acute{e}$rsic index ($ n \sim 1$) derived from a 2-D imaging decomposition
of the HST WFPC2 image taken in the $I_{F814}$ filter, the identified system is
consistent with a massive disk galaxy at z>3. If confirmed, it would be one of
the most distant massive disk galaxies known so far.
