PROBLEM TO BE SOLVED: To provide a device for diagnosing catalyst degradation, in which a cost increase is suppressed and which can determine to what extent a catalyst has degraded.SOLUTION: The device for diagnosing the catalyst degradation includes: an oxidation catalyst 23 which is provided in an exhaust passage of an engine 10; a first NOx sensor 41 which is provided on the downstream side of the oxidation catalyst 23 and which detects a first NOx value as the amount of NOx contained in exhaust gas after passage through the oxidation catalyst 23; a second NOx value obtaining means 32 for obtaining a second NOx value as the amount of NOx contained in exhaust gas immediately after it is discharged from the engine 10; an NOvalue obtaining means 33 for obtaining a value of NOcontained in the exhaust gas after the passage through the oxidation catalyst 23, on the basis of the first NOx value detected by the first NOx sensor 41 and the second NOx value obtained by the second NOx value obtaining means 32; and a degradation determining means 34 for determining the degree of the degradation of the oxidation catalyst 23, on the basis of the NOvalue obtained by the NOvalue obtaining means 33 and a temperature of the oxidation catalyst 23.