A failure diagnosis device for use with an emission control system comprising a particulate filter that is to be placed in an exhaust conduit of an internal combustion engine and is configured to trap PM in exhaust gas; an exhaust gas purification device that is to be placed upstream of the particulate filter in the exhaust conduit and is configured to purify the exhaust gas by utilizing a non-combusted fuel component included in the exhaust gas; and a supplier (3, 6) that is configured to perform in-cylinder rich control of changing an air-fuel ratio of an air-fuel mixture subjected to combustion in the internal combustion engine to a rich air-fuel ratio which is lower than a stoichiometric air-fuel ratio, so as to supply the non-combusted fuel component to the exhaust gas purification device, the failure diagnosis device comprising: a PM sensor that is adapted to be provided downstream of a particulate filter and configured to detect an amount of PM flowing out of the particulate filter, the PM sensor having a sensor element that includes electrodes (514, 542) opposed to each other across an insulating layer and a heater that is configured to heat the sensor element, the PM sensor being configured to output an electric signal relating to an amount of PM deposited between the electrodes (514, 542) under application of a predetermined voltage to the sensor element; and a controller configured to diagnose a failure of the particulate filter, based on an output value of the PM sensor, wherein the controller comprises: a measurement unit (6, S104-S110) configured to perform a sensor recovery process that controls the heater to heat the sensor element to a temperature that allows for oxidation of PM and thereby oxidizes and removes the PM deposited between the electrodes (514, 542), and a measurement process that starts application of the predetermined voltage to the sensor element after termination of the sensor recovery process and measures an output value of the PM sensor when a predetermined time period has elapsed since start of application of the predetermined voltage; and a diagnosis unit (6, S111-S113) configured to compare the output value of the PM sensor measured by the measurement unit (6, S104-S110) with a predefined reference value and diagnose a failure of the particulate filter, wherein the measurement unit (6, S104-S110) is configured to perform the sensor recovery process during the in-cylinder rich control or triggered by termination of the in-cylinder rich control and subsequently to perform the measurement process after termination of the in-cylinder rich control and the sensor recovery process. a PM sensor that is adapted to be provided downstream of a particulate filter and configured to detect an amount of PM flowing out of the particulate filter, the PM sensor having a sensor element that includes electrodes (514, 542) opposed to each other across an insulating layer and a heater that is configured to heat the sensor element, the PM sensor being configured to output an electric signal relating to an amount of PM deposited between the electrodes (514, 542) under application of a predetermined voltage to the sensor element; and a controller configured to diagnose a failure of the particulate filter, based on an output value of the PM sensor, wherein the controller comprises: a measurement unit (6, S104-S110) configured to perform a sensor recovery process that controls the heater to heat the sensor element to a temperature that allows for oxidation of PM and thereby oxidizes and removes the PM deposited between the electrodes (514, 542), and a measurement process that starts application of the predetermined voltage to the sensor element after termination of the sensor recovery process and measures an output value of the PM sensor when a predetermined time period has elapsed since start of application of the predetermined voltage; and a diagnosis unit (6, S111-S113) configured to compare the output value of the PM sensor measured by the measurement unit (6, S104-S110) with a predefined reference value and diagnose a failure of the particulate filter, wherein the measurement unit (6, S104-S110) is configured to perform the sensor recovery process during the in-cylinder rich control or triggered by termination of the in-cylinder rich control and subsequently to perform the measurement process after termination of the in-cylinder rich control and the sensor recovery process. a measurement unit (6, S104-S110) configured to perform a sensor recovery process that controls the heater to heat the sensor element to a temperature that allows for oxidation of PM and thereby oxidizes and removes the PM deposited between the electrodes (514, 542), and a measurement process that starts application of the predetermined voltage to the sensor element after termination of the sensor recovery process and measures an output value of the PM sensor when a predetermined time period has elapsed since start of application of the predetermined voltage; and a diagnosis unit (6, S111-S113) configured to compare the output value of the PM sensor measured by the measurement unit (6, S104-S110) with a predefined reference value and diagnose a failure of the particulate filter, wherein the measurement unit (6, S104-S110) is configured to perform the sensor recovery process during the in-cylinder rich control or triggered by termination of the in-cylinder rich control and subsequently to perform the measurement process after termination of the in-cylinder rich control and the sensor recovery process.