A sealed alkaline storage battery using, as a positive electrode active material, nickel oxyhydroxide including Mn as a solid-solution element and having a &ggr; ratio of 65 through 100%; a sealed alkaline storage battery using, as a positive electrode active material, nickel oxyhydroxide including as an additive or coated with a rare earth element and/or a rare earth compound in a ratio measured based on the rare earth element of 0.05 through 5 wt %; and a sealed alkaline storage battery including, as a positive electrode active material, nickel oxyhydroxide having a half-width of a peak in a lattice plane (003) in an X-ray diffraction pattern of 0.8° or more. The pressure within the battery is not largely increased for a long period of charge-discharge cycles, and hence, the electrolyte hardly leaks.