Enabled is reliable detection of abnormality in a NOx emission control system.Disclosed is a method for detecting abnormality in an exhaust emission control device with a reducing agent (urea water 17) being added to selective reduction catalyst 10 incorporated in an exhaust pipe 9 so as to reduce and purify NOx. Temperature of the catalyst during an operation period is monitored to record a cumulative time for each of temperature zones. On the basis of the recorded cumulative time for each of the temperature zones, a deterioration coefficient of NOx reduction performance is determined for each of the temperature zones. A standard NOx reduction ratio predetermined for each of the temperature zones is multiplied by the determined deterioration coefficient for each of the temperature zones to update the standard NOx reduction ratio. The updated standard NOx reduction ratio is used in a next operation to compare an actually measured NOx reduction ratio with the standard NOx reduction ratio on the measured temperature zone. It is determined that there is abnormality when deviation of the actually measured NOx reduction ratio from the standard NOx reduction ratio does not fall within a required range.