  We report on the discovery of a very narrow-line star forming object beyond
redshift of 5. Using the prime-focus camera, Suprime-Cam, on the 8.2 m Subaru
telescope together with a narrow-passband filter centered at $\lambda_{\rm c}$
= 8150 \AA with passband of $\Delta\lambda$ = 120 \AA, we have obtained a very
deep image of the field surrounding the quasar SDSSp J104433.04$-$012502.2 at a
redshift of 5.74. Comparing this image with optical broad-band images, we have
found an object with a very strong emission line. Our follow-up optical
spectroscopy has revealed that this source is at a redshift of
$z=5.655\pm0.002$, forming stars at a rate $\sim 13 ~ h_{0.7}^{-2} ~ M_\odot$
yr$^{-1}$. Remarkably, the velocity dispersion of Ly$\alpha$-emitting gas is
only 22 km s$^{-1}$. Since a blue half of the Ly$\alpha$ emission could be
absorbed by neutral hydrogen gas, perhaps in the system, a modest estimate of
the velocity dispersion may be $\gtrsim$ 44 km s$^{-1}$. Together with a linear
size of 7.7 $h_{0.7}^{-1}$ kpc, we estimate a lower limit of the dynamical mass
of this object to be $\sim 2 \times 10^9 M_\odot$. It is thus suggested that
LAE J1044$-$0123 is a star-forming dwarf galaxy (i.e., a subgalactic object or
a building block) beyond redshift 5 although we cannot exclude a possibility
that most Ly$\alpha$ emission is absorbed by the red damping wing of neutral
intergalactic matter.
