An object of the present invention is to provide a catalyst which decomposes a peroxide effectively and economically under a high temperature while suppressing generation of free radicals, and the present invention provides a peroxide decomposition catalyst containing a base metal atom, wherein a value A indicating a free radical generation amount represented by the (equation 1) is not more than 0.20, and a value B indicating a reaction rate represented by the (equation 2), which can be easily applied to utility such as an agent for preventing deterioration of a polymer electrolyte-type fuel cell and a water electrolysis apparatus, and an antioxidant for medicaments, agrochemicals and foods. A=(Mw(S)/Mw)−1  (equation 1) (wherein Mw is a weight average molecular weight of poly(sodium 4-styrenesulfonate) after a hydrogen peroxide decomposition test in the test at 80° C. in the presence of poly(sodium 4-styrenesulfonate), and Mw(S) is a weight average molecular weight of poly(sodium 4-styrenesulfonate) before the test) B=N(PO)/N(cat)  (equation 2) (wherein N(po) is a mole number of hydrogen peroxide decomposed per 20 minutes in a hydrogen peroxide decomposition test in the (equation 1), and N(cat) is a mole number per metal atom of a catalyst used).