Spectroscopy and Multi-color Photometry of U Scorpii at the Earliest Stage of 2010 Outburst

We have performed our spectroscopic and multi-color photometric observations of the recurrent nova U Scorpii at the earliest stage of the 2010 outburst. 0.37 days after the discovery of the outburst, we can see broad and prominent emission lines of Balmer series, He I, N II, N III, O I and Mg II on the spectra. The FWHM of H$\alpha$ line yields an expansion velocity of approximately 6200 km/s. This line also accompanies a blue shifted absorption line (so called P Cygni profile). 1.37 days after the discovery, H$\alpha$ line shows a nearly flat-topped profile in contrast to the previous day. From our multi-color photometry, we can see its rapid decline (one magnitude per day) of the brightness in each color band.