A reducing agent supplying device includes reaction container (20), ozone generator (30), air pump (30p), ozone-containing air pipe (26), compressed air pipe (25), switching device (25v, 26v), and switching controller (41 a). The reaction container defines a reaction chamber (20a) in which reducing agent is reformed. The ozone generator generates ozone from oxygen in air, being supplied by the air pump. Ozone-containing air flows through the ozone-containing air pipe toward the reaction chamber. A portion of compressed air flows through the compressed air pipe toward the reaction chamber. The switching device switches between an air pump mode, in which the ozone-containing air is supplied into the reaction chamber, and a supercharging mode, in which the compressed air is supplied into the reaction chamber. The switching controller controls the switching device based on a temperature of the reducing catalyst and an exhaust pressure in the exhaust passage.