An object of the invention is to suppress a reduction in accuracy of abnormality diagnosis with an abnormality diagnosis device for an exhaust gas control apparatus that performs the abnormality diagnosis regarding the exhaust gas control apparatus by using the amount of NOx flowing into an SCR catalyst as a parameter. In the abnormality diagnosis device for an exhaust gas control apparatus according to the invention provided with an exhaust gas control apparatus including the SCR catalyst, a supply device supplying ammonia to the exhaust gas control apparatus, an EGR device allowing some of exhaust gas to flow back to an intake passage from an exhaust passage further downstream than the supply device, acquiring means for acquiring a NOx inflow amount as the amount of NOx flowing into the exhaust gas control apparatus, and diagnostic means for diagnosing an abnormality of the exhaust gas control apparatus by using the NOx inflow amount acquired by the acquiring means as a parameter, the abnormality diagnosis regarding the exhaust gas control apparatus by the diagnostic means is prohibited in a case where the amount of the ammonia allowed to flow back along with the exhaust gas by the EGR device exceeds an upper limit value.