An exhaust gas purification device, comprising: a first catalyst which is provided on an exhaust gas passageway of an internal combustion engine, and has a function which adsorbs ammonia and selectively purifies NO x in exhaust gas using the ammonia; a second catalyst which is provided on the exhaust gas passageway on a downstream side of the first catalyst, and has a function which adsorbs the ammonia flowing out from the first catalyst and selectively purifies the NO x flowing out from the first catalyst using the ammonia; a reducing agent supply device which supplies a liquid reducing agent derived from the ammonia into the exhaust gas passageway at an upstream side of the first catalyst; upstream side NO x concentration detection means for detecting NO x concentration at the upstream side of the first catalyst; downstream side NO x concentration detection means for detecting NO x concentration at the downstream side of the second catalyst; a purification rate diagnosis unit which diagnoses an NO x purification rate in the exhaust gas purification device, wherein the NO x purification rate is calculated using the NO x concentration detected at the upstream side of the first catalyst and the NO x concentration detected at the downstream side of the second catalyst; and an abnormality determination unit configured to specify abnormality of the second catalyst based on a diagnostic result of the NO x purification rate obtained at the time of a high load operation of the internal combustion engine and a diagnostic result of the NO x purification rate obtained at the time of a low load operation of the internal combustion engine. a first catalyst which is provided on an exhaust gas passageway of an internal combustion engine, and has a function which adsorbs ammonia and selectively purifies NO x in exhaust gas using the ammonia; a second catalyst which is provided on the exhaust gas passageway on a downstream side of the first catalyst, and has a function which adsorbs the ammonia flowing out from the first catalyst and selectively purifies the NO x flowing out from the first catalyst using the ammonia; a reducing agent supply device which supplies a liquid reducing agent derived from the ammonia into the exhaust gas passageway at an upstream side of the first catalyst; upstream side NO x concentration detection means for detecting NO x concentration at the upstream side of the first catalyst; downstream side NO x concentration detection means for detecting NO x concentration at the downstream side of the second catalyst; a purification rate diagnosis unit which diagnoses an NO x purification rate in the exhaust gas purification device, wherein the NO x purification rate is calculated using the NO x concentration detected at the upstream side of the first catalyst and the NO x concentration detected at the downstream side of the second catalyst; and an abnormality determination unit configured to specify abnormality of the second catalyst based on a diagnostic result of the NO x purification rate obtained at the time of a high load operation of the internal combustion engine and a diagnostic result of the NO x purification rate obtained at the time of a low load operation of the internal combustion engine.