An abnormality detection device for an exhaust gas purification apparatus, for detecting an abnormality of the exhaust gas purification apparatus comprising: a filter which is provided for an exhaust gas passage of an internal combustion engine and which traps particulate matter contained in an exhaust gas, the filter including a selective catalytic reduction NOx catalyst carried thereon; a reducing agent addition valve which is provided for the exhaust gas passage on an upstream side from the filter and which adds a reducing agent into the exhaust gas; and a diffusing device which is provided for the exhaust gas passage between the filter and the reducing agent addition valve and which diffuses the reducing agent contained in the exhaust gas, the abnormality detection device for the exhaust gas purification apparatus comprising: a judging unit (S107, S108, S109) configured to judge that which one of abnormalities of any clogging-up of the filter caused by the particulate matter and any abnormality of the diffusing device occurs, on the basis of a difference between a NOx purification rate of the selective catalytic reduction NOx catalyst carried on the filter and a predetermined NOx purification rate, when a difference, which is provided between an exhaust gas pressure difference as a difference between an exhaust gas pressure on an upstream side from the reducing agent addition valve and an exhaust gas pressure on a downstream side from the filter and a predetermined exhaust gas pressure difference, exceeds a certain amount, and a difference, which is provided between a particulate matter amount flowing out from the filter and a predetermined outflow particulate matter amount, is not more than a predetermined threshold value. a filter which is provided for an exhaust gas passage of an internal combustion engine and which traps particulate matter contained in an exhaust gas, the filter including a selective catalytic reduction NOx catalyst carried thereon; a reducing agent addition valve which is provided for the exhaust gas passage on an upstream side from the filter and which adds a reducing agent into the exhaust gas; and a diffusing device which is provided for the exhaust gas passage between the filter and the reducing agent addition valve and which diffuses the reducing agent contained in the exhaust gas, the abnormality detection device for the exhaust gas purification apparatus comprising: a judging unit (S107, S108, S109) configured to judge that which one of abnormalities of any clogging-up of the filter caused by the particulate matter and any abnormality of the diffusing device occurs, on the basis of a difference between a NOx purification rate of the selective catalytic reduction NOx catalyst carried on the filter and a predetermined NOx purification rate, when a difference, which is provided between an exhaust gas pressure difference as a difference between an exhaust gas pressure on an upstream side from the reducing agent addition valve and an exhaust gas pressure on a downstream side from the filter and a predetermined exhaust gas pressure difference, exceeds a certain amount, and a difference, which is provided between a particulate matter amount flowing out from the filter and a predetermined outflow particulate matter amount, is not more than a predetermined threshold value. a judging unit (S107, S108, S109) configured to judge that which one of abnormalities of any clogging-up of the filter caused by the particulate matter and any abnormality of the diffusing device occurs, on the basis of a difference between a NOx purification rate of the selective catalytic reduction NOx catalyst carried on the filter and a predetermined NOx purification rate, when a difference, which is provided between an exhaust gas pressure difference as a difference between an exhaust gas pressure on an upstream side from the reducing agent addition valve and an exhaust gas pressure on a downstream side from the filter and a predetermined exhaust gas pressure difference, exceeds a certain amount, and a difference, which is provided between a particulate matter amount flowing out from the filter and a predetermined outflow particulate matter amount, is not more than a predetermined threshold value.