In the excimer lamp according to the present invention, a flat discharge vessel having a substantially rectangular cross-sectional shape and comprising a pair of planar parts and a pair of side-surface parts has a pair of external electrodes disposed on the respective outer surfaces of the planar parts. The end parts of the external electrodes are provided with an auxiliary electrode extending to a region that is made smaller than the distance between the planar parts. A lead that supplies electricity to the external electrode is connected to the auxiliary electrode in the region that is made smaller than the distance between the planar parts.