Provided is a solid acid catalyst for an oxidation reaction in the presence of both oxygen and ozone, which is used when a substrate is being oxidized in the presence of both oxygen and ozone, wherein the substrate can be oxidized with a high conversion rate. This solid acid catalyst for an oxidation reaction in the presence of both oxygen and ozone is used when a substrate (A) is being oxidized in the presence of both oxygen and ozone and is characterized in that a transition metal is supported in the form of an elemental substance, a compound, or an ion, on a carrier that has, at least on the surface thereof, a strong acid or super acid having a Hammett acidity function (H0) of -9 or lower. The carrier is preferably pellets or a powdered or granular material formed of a fluorinated sulfonic acid resin, or a carrier having a coating of a fluorinated sulfonic acid resin formed on a solid surface thereof.