An injection amount of a reducing agent such as urea water is properly and adequately controlled to keep a NOx reduction ratio as high as possible. Disclosed is a method for controlling an exhaust emission control device wherein urea water 17 (reducing agent) is added to selective reduction catalyst 10 incorporated in an exhaust pipe 9 so as to reduce and purify NOx. A first-order lag response model corresponding to the exhaust temperature upstream of the catalyst 10 is used to estimate catalyst temperature for each of cells into which the catalyst is minutely divided. Cell volumes for each of temperature zones are summed up on the basis of the estimated temperatures for the cells. The cell volume summation for each of the temperature zones is divided by the whole volume of the catalyst to determine temperature distribution volume ratio. The temperature distribution volume ratio for each of the temperature zones is multiplied by a reference injection amount of the urea water 17 which is determined in consideration to an operation status of a current engine 1 on the assumption that the catalyst temperatures are all within the temperature zone. The calculated values for the respective temperature zones are summed up into a directive injection amount of the urea water 17.