In a failure diagnosis device of an emission control system that utilizes an electrode-based PM sensor (54) to diagnose a failure of a particulate filter (51), an object of the invention is to suppress reduction of accuracy of diagnosis of a failure due to in-cylinder rich control. The failure diagnosis device of the emission control system performs a measurement process. The measurement process includes a sensor recovery process of removing PM depositing between the electrodes of the electrode-based PM sensor (54), a process of starting application of the predetermined voltage to the electrodes of the PM sensor after completion of the sensor recovery process, and a process of obtaining an output value of the PM sensor after elapse of a predetermined time period since the start of application of the predetermined voltage to the electrodes of the PM sensor. The failure diagnosis device diagnoses a failure of the particulate filter, based on an output value of the PM sensor obtained by the measurement process. The failure diagnosis device does not perform the measurement process on prediction that in-cylinder rich control is performed in the predetermined time period.