To remove NOx in exhaust gas over a load area of an engine and efficiently reduce NOx in exhaust gas discharged to the atmosphere.

EGR gas is circulated to an air intake passage 12 from an exhaust gas passage 13 of a diesel engine 11 through an EGR pipe 16. The quantity of EGR gas flowing in the EGR pipe 16 is regulated by an EGR valve 16a. EGR gas flowing in the EGR pipe is cooled by engine cooling water circulating in an EGR cooler 16b. A selective reduction type catalyst 17 is provided in the exhaust gas passage. A liquid supply means 23 injects urea-based liquid from a liquid injection nozzle 23a inserted in the exhaust gas passage at the exhaust gas upstream side of the selective reduction type catalyst. An ammonia slip prevention catalyst 18 is provided in the exhaust gas passage at the exhaust gas downstream side of the selective reduction type catalyst. A controller 28 controls the EGR valve and the liquid supply means based on the detection output of an operation state detecting means 25 detecting the operation state of the engine.