Disclosed is a method for detecting abnormality in a reducing agent 17 replenished to a reducing agent tank 14 in an exhaust emission control device for reduction and removal of NOx through addition of the reducing agent 17 from the tank 14 to a selective reduction catalyst 10 incorporated in an exhaust pipe 9. Presence or absence of NH3 slip is determined when a lowering of NOx removal rate is detected. When the presence of the NH3 slip is detected, it is determined that the selective reduction catalyst 10 is deteriorated; when the absence of the NH3 slip is determined, it is determined that a dilute reducing agent or/and a material other than the reducing agent are replenished into the tank.