Subject To enable an appropriate operation of an exhaust gas purifying apparatus which uses urea or the like as a reducing agent, and further, to enable the stable concentration detection and the stable supply of the reducing agent during the cold months and also to prevent the generation of ammonia series gas during the severe hot months.Means for solving the SubjectAn exhaust gas purifying apparatus in an engine 1 is for supplying the urea water in a storage tank 20 to a reduction catalyst 3 disposed in an exhaust system. In this exhaust gas purifying apparatus, a concentration detecting device 60 is disposed in the storage tank 20, and the detected concentration of the urea water is adopted as basic information for the supply control of the reducing agent. Further, a heating device heating the reducing agent in the storage tank 20 and a cooling device cooling this reducing agent are disposed, to heat, preferably, the reducing agent in the vicinity of a detecting section 61 of the concentration detecting device 60 and the reducing agent in the vicinity of an inlet port 12 of a supply pipe 12.