A process of causing an engine (10) to perform high-temperature exhaust driving (D1) after urea water from an urea water injection valve (33) is not supplied to an SCR catalyst (34), and a process of causing the engine (10) to perform high-flow-rate exhaust driving (D2) after a flow rate of exhaust gas from a cylinder interior (13) is not reduced are performed on an ECU (40). A process of determining deterioration or malfunction of upstream and downstream NOx sensors (47 and 48) based on detected values (Ca and Cb) acquired when the engine speed (Na) reaches a determination speed (Nx) is performed on a vehicle external computer (52).