An electrode comprising an active material-containing layer which comprises an active material, wherein 1) the active material comprises a plurality of primary particles comprising a niobium-titanium composite oxide, 2) the average crystallite diameter of the plurality of primary particles is 105-200 nm, 3) the average particle size of the plurality of primary particles is 0.1-5 μm, which is based on a particle size distribution chart according to a laser diffraction scattering method, 4) in 100 primary particles among the plurality of primary particles the average value (FU ave) of a roughness shape coefficient according to Formula below is < 0.70: FU = f f c = 4 πa l 2 wherein l is the outer circumference length of the projected cross-section of each of the 100 primary particles, and a is a cross-sectional area in the projected cross-section of each of the 100 primary particles; the 100 primary particles are selected from the plurality of primary particles in a discretional manner by a method comprising: - preparing two electrode pieces by cutting the electrode; - separating the active material from one of the two electrode pieces; - measuring the particle size distribution of the active material by a laser diffraction scattering method to determine the average particle size of the plurality of primary particles; - observing a cross section of the other of the two electrode pieces by scanning electron microscope with a magnification of 5,000-20,000 times such that a center portion of the cross section in a short direction of the cross section is observed at 100 points at regular intervals along a longitudinal direction of the cross section from an end of the electrode; - selecting the 100 primary particles each having a particle size of 0.2-4 times the average particle size at the observed 100 points, respectively. 1) the active material comprises a plurality of primary particles comprising a niobium-titanium composite oxide, 2) the average crystallite diameter of the plurality of primary particles is 105-200 nm, 3) the average particle size of the plurality of primary particles is 0.1-5 μm, which is based on a particle size distribution chart according to a laser diffraction scattering method, 4) in 100 primary particles among the plurality of primary particles the average value (FU ave) of a roughness shape coefficient according to Formula below is < 0.70: FU = f f c = 4 πa l 2 wherein l is the outer circumference length of the projected cross-section of each of the 100 primary particles, and a is a cross-sectional area in the projected cross-section of each of the 100 primary particles; the 100 primary particles are selected from the plurality of primary particles in a discretional manner by a method comprising: - preparing two electrode pieces by cutting the electrode; - separating the active material from one of the two electrode pieces; - measuring the particle size distribution of the active material by a laser diffraction scattering method to determine the average particle size of the plurality of primary particles; - observing a cross section of the other of the two electrode pieces by scanning electron microscope with a magnification of 5,000-20,000 times such that a center portion of the cross section in a short direction of the cross section is observed at 100 points at regular intervals along a longitudinal direction of the cross section from an end of the electrode; - selecting the 100 primary particles each having a particle size of 0.2-4 times the average particle size at the observed 100 points, respectively. - preparing two electrode pieces by cutting the electrode; - separating the active material from one of the two electrode pieces; - measuring the particle size distribution of the active material by a laser diffraction scattering method to determine the average particle size of the plurality of primary particles; - observing a cross section of the other of the two electrode pieces by scanning electron microscope with a magnification of 5,000-20,000 times such that a center portion of the cross section in a short direction of the cross section is observed at 100 points at regular intervals along a longitudinal direction of the cross section from an end of the electrode; - selecting the 100 primary particles each having a particle size of 0.2-4 times the average particle size at the observed 100 points, respectively.