Provided is a separator having excellent tearing strength, denseness, and impedance performance, and an aluminum electrolytic capacitor provided with the separator. The present invention constitutes a separator including beatable regenerated cellulose fibers, the separator having a CSF value X[ml] and tear index Y[mN·m2/g] within the ranges satisfying the following formulae. The present invention also constitutes an aluminum electrolytic capacitor in which at least one of the separators is used, the capacitor feeing formed by interposing the separator between an anode and a cathode. 0≤X≤300   Formula 1 15≤Y≤100   Formula 2 Y≥0.175X−2.5   Formula 3