A catalyst determining device includes an oxidation catalyst disposed in an outlet passage in an engine, the oxidation catalyst including an adsorption substance for adsorbing and desorbing an exhaust component in exhaust and a catalytic substance capable of oxidizing the exhaust component, a calculating unit for calculating a difference in oxygen concentration between upstream and downstream of the oxidation catalyst, and a determination unit for determining the deterioration of the oxidation catalyst through comparison of a plurality of the differences in oxygen concentration obtained at different temperatures of the oxidation catalyst by the calculating unit.