An active material comprising active material particles containing orthorhombic Na-containing niobium titanium composite oxide, and satisfying the following formula: 1 ≤ A 5 / A 0 where A5 is a mole content ratio of a Li mole content L5 to a total of a Ti mole content T5 and a Nb mole content N5, and each of the Li mole content L5, the Ti mole content T5, and the Nb mole content N5 is a value according to an X-ray photoelectron spectroscopy measurement, obtained at a depth of 5 nm from surfaces of the active material particles, and A0 is a mole content ratio of a Li mole content L0 to a total of a Ti mole content T0 and a Nb mole content N0, and each of the Li mole content L0, the Ti mole content T0, and the Nb mole content N0 is a value according to the X-ray photoelectron spectroscopy measurement, obtained at the surfaces of the active material particles, wherein the orthorhombic Na-containing niobium titanium composite oxide is represented by the general formula Li 2+v Na 2-w M1 x Ti 6-y-z Nb y M2 z O 14+δ, M1 is at least one element selected from the group consisting of Cs, K, Sr, Ba, Mg, and Ca, M2 is at least one element selected from the group consisting of Zr, Sn, V, Ta, Mo, W, Fe, Co, Mn, and Al, and 0 ≤ v ≤ 4, 0 < w < 2, 0 ≤ x ≤ 0.2, 0 ≤ y ≤ 1, 0 ≤ z ≤ 0.3, and -0.5 ≤ δ ≤ 0.5 hold, the mole content ratio A0 is determined from the X-ray photoelectron spectroscopy measurement before ion etching, and the mole content ratio A5 is determined from the X-ray photoelectron spectroscopy measurement after the ion etching, which is performed for 0.8 min by Ar ions at an accelerating voltage of 2 kV, and an etching rate is set to 6.1 nm/min (value based on SiO 2), and the X-ray photoelectron spectroscopy measurement is performed under an excitation X-Ray source of single crystal spectroscopy Al-Kα (1486.6 eV) and a photoelectron detection angle of 45 degrees. where A5 is a mole content ratio of a Li mole content L5 to a total of a Ti mole content T5 and a Nb mole content N5, and each of the Li mole content L5, the Ti mole content T5, and the Nb mole content N5 is a value according to an X-ray photoelectron spectroscopy measurement, obtained at a depth of 5 nm from surfaces of the active material particles, and A0 is a mole content ratio of a Li mole content L0 to a total of a Ti mole content T0 and a Nb mole content N0, and each of the Li mole content L0, the Ti mole content T0, and the Nb mole content N0 is a value according to the X-ray photoelectron spectroscopy measurement, obtained at the surfaces of the active material particles, wherein the orthorhombic Na-containing niobium titanium composite oxide is represented by the general formula Li 2+v Na 2-w M1 x Ti 6-y-z Nb y M2 z O 14+δ, M1 is at least one element selected from the group consisting of Cs, K, Sr, Ba, Mg, and Ca, M2 is at least one element selected from the group consisting of Zr, Sn, V, Ta, Mo, W, Fe, Co, Mn, and Al, and 0 ≤ v ≤ 4, 0 < w < 2, 0 ≤ x ≤ 0.2, 0 ≤ y ≤ 1, 0 ≤ z ≤ 0.3, and -0.5 ≤ δ ≤ 0.5 hold, the mole content ratio A0 is determined from the X-ray photoelectron spectroscopy measurement before ion etching, and the mole content ratio A5 is determined from the X-ray photoelectron spectroscopy measurement after the ion etching, which is performed for 0.8 min by Ar ions at an accelerating voltage of 2 kV, and an etching rate is set to 6.1 nm/min (value based on SiO 2), and the X-ray photoelectron spectroscopy measurement is performed under an excitation X-Ray source of single crystal spectroscopy Al-Kα (1486.6 eV) and a photoelectron detection angle of 45 degrees.