According to one approach, an electrode (3) is provided. The electrode (3) includes an active material-containing layer (3b). The active material-containing layer (3b) includes an Na-containing niobium-titanium composite oxide having an orthorhombic crystal structure. The active material-containing layer (3b) satisfies I 2 /I 1 ≥ 1. I 1 is an intensity of a peak P 1 appearing in a binding energy range of 289 eV to 292 eV in an X-ray photoelectron spectroscopy spectrum of the active material-containing layer (3b). I 2 is an intensity of a peak P 2 appearing in a binding energy range of 283 eV to 285 eV in the X-ray photoelectron spectroscopy spectrum of the active material-containing layer (3b).