A failure diagnosis apparatus for an exhaust gas control apparatus, the exhaust gas control apparatus comprising a selective reduction-type catalyst supported by a particulate filter, the failure diagnosis apparatus comprising: a NO x sensor arranged on a downstream side from the exhaust gas control apparatus, the NOx sensor configured to detect an amount of NO x contained in exhaust gas; and an electronic control unit configured to a) estimate a PM accumulation amount as an amount of particulate matter accumulated in the exhaust gas control apparatus, b) estimate a NO 2 ratio as a ratio of nitrogen dioxide in NO x flowing into the exhaust gas control apparatus, c) compute a reference NO x purification rate as a NO x purification rate pertaining to a case where the exhaust gas control apparatus is normal by using, as parameters, said estimated PM accumulation amount as the PM accumulation amount estimated and said estimated NO 2 ratio as the NO 2 ratio estimated, d) compute an actual NO x purification rate as a real NO x purification rate of the exhaust gas control apparatus by using the amount of the NO x detected by the NO x sensor as a parameter, and e) determine that a selective reduction function of the selective reduction-type catalyst fails on condition of a difference between the reference NO x purification rate and the actual NO x purification rate exceeding a threshold. a NO x sensor arranged on a downstream side from the exhaust gas control apparatus, the NOx sensor configured to detect an amount of NO x contained in exhaust gas; and an electronic control unit configured to a) estimate a PM accumulation amount as an amount of particulate matter accumulated in the exhaust gas control apparatus, b) estimate a NO 2 ratio as a ratio of nitrogen dioxide in NO x flowing into the exhaust gas control apparatus, c) compute a reference NO x purification rate as a NO x purification rate pertaining to a case where the exhaust gas control apparatus is normal by using, as parameters, said estimated PM accumulation amount as the PM accumulation amount estimated and said estimated NO 2 ratio as the NO 2 ratio estimated, d) compute an actual NO x purification rate as a real NO x purification rate of the exhaust gas control apparatus by using the amount of the NO x detected by the NO x sensor as a parameter, and e) determine that a selective reduction function of the selective reduction-type catalyst fails on condition of a difference between the reference NO x purification rate and the actual NO x purification rate exceeding a threshold. a) estimate a PM accumulation amount as an amount of particulate matter accumulated in the exhaust gas control apparatus, b) estimate a NO 2 ratio as a ratio of nitrogen dioxide in NO x flowing into the exhaust gas control apparatus, c) compute a reference NO x purification rate as a NO x purification rate pertaining to a case where the exhaust gas control apparatus is normal by using, as parameters, said estimated PM accumulation amount as the PM accumulation amount estimated and said estimated NO 2 ratio as the NO 2 ratio estimated, d) compute an actual NO x purification rate as a real NO x purification rate of the exhaust gas control apparatus by using the amount of the NO x detected by the NO x sensor as a parameter, and e) determine that a selective reduction function of the selective reduction-type catalyst fails on condition of a difference between the reference NO x purification rate and the actual NO x purification rate exceeding a threshold.