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.
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