Abstract: A powder contains a niobium complex and lithium, an amount of niobium being 25 mass % or more and 75 mass % or less, a proportion of niobium in metal elements of the powder is 0.775 or more and 0.950 or less in terms of mass ratio. When the powder is dissolved in 8 times its mass of water at 25° C., a niobium content contained in a filtrate thereof is 80 mass % or more of an amount of niobium contained in the powder before dissolution. The powder is obtained by mixing a niobium compound, a lithium compound, an alkali, hydrogen peroxide, and water to obtain an aqueous solution containing a niobium complex and lithium and then drying the solution at a temperature equal to or lower than a decomposition temperature of the niobium complex. The powder is suitable for preparing a lithium niobate precursor solution for coating positive electrode active material particles.