An electrode/electrolyte interface is obviously one of the most dominant factors controlling a battery performance. In the present work, a formation of a tin negative electrode/electrolyte interface (SEI) was investigated by an in situ X-ray reflectivity measurement. SEI was formed even at a high voltage of about 2.5 V vs Li/Li+. This voltage is much higher than the value of about 1.5 V in the previous works. Thus, SEI in the present work is clearly different from that formed by reductive decomposition of an electrolyte and/or reduction of Sn oxide. Its structure was seemingly an amorphous solid containing a quite amount of fluoride, which grows up to 150-200 nm thick by keeping for 1 h at about 2.5-2.6 V.