A catalyst that is not only capable of efficiently treating CO even at a low exhaust gas temperature, but also capable of exerting favorable CO purification efficiency in a low-temperature exhaust gas even in a case of being exposed for a long time to an engine exhaust gas that is a high temperature and contains HC, CO, NOx, water vapor and the like; and an exhaust gas treatment technique are described. The catalyst for purifying exhaust gas contains: a noble metal; an oxide containing as a base material A at least two kinds of elements selected from the group consisting of aluminum, zirconium and titanium; and an oxide containing as a base material B at least one kind of element selected from the group consisting of silicon, cerium, praseodymium and lanthanum; in which the base material A and the base material B satisfy a specific formula.