An active material for a battery includes an electrochemically reversibly oxidizable and reducible base material selected from the group consisting of a metal, a lithium-containing alloy, a sulfur-based compound, and a compound that can reversibly form a lithium-containing compound by a reaction with lithium ions and a surface-treatment layer formed on the base material and comprising a compound of the formula MXO.sub.k, wherein M is at least one element selected from the group consisting of an alkali metal, an alkaline earth metal, a group 13 element, a group 14 element, a transition metal, and a rare-earth element, X is an element that is capable of forming a double bond with oxygen, k is a numerical value in the range of 2 to 4.