A temperature-sensitive type urea sensor 74 is adopted for use in an engine exhaust gas purifying apparatus. The magnitude of convection of urea aqueous solution generated in a storage tank 41 is determined based on temperature information detected by the urea sensor 74 . For detection of abnormality, a detected concentration Dn is adopted when the percentage GRDt of temperature change of urea aqueous solution is small, so that convection is small and it is determined not to affect accurate detection of an abnormality.