  We present deep optical spectroscopy of an extended Ly$\alpha$ emission-line
blob located in an over-dense region at redshift $z \approx 3.1$; `blob 1' of
Steidel et al. (2000). The origin of such Ly$\alpha$ blobs has been debated for
some time; two of the most plausible models are (1) that it comes from a
dust-enshrouded, extreme starburst galaxy with a large-scale galactic outflow
(superwind/hyperwind) or (2) that it is the cooling radiation of proto-galaxies
in dark matter halos. Examination of the kinematic properties of the Ly$\alpha$
emission-line gas should allow us to determine its nature. With this
motivation, we performed optical spectroscopy of `blob 1' using the Subaru
Telescope, and found that its kinematic properties can be well explained in
terms of superwind activity.
