There are provided a trouble diagnosis device and a trouble diagnosis method for an exhaust gas purification system in which the presence or absence of a trouble of the exhaust gas purification system such as abnormality of a control system for the exhaust gas purification system, catalyst deterioration or the like can be determined with high precision, and the reliability of the exhaust gas purification system can be enhanced.The device includes upstream-side NOx flow rate calculating means, downstream-side NOx flow rate calculating means, reduction condition determining means for determining whether at least one condition for performing normal reduction of NOx is satisfied, upstream-side NOx flow amount calculating means for integrating the upstream-side NOx flow amount to calculate the amount of NOx passing through the upstream side of the catalyst within a predetermined time when the condition concerned is determined to be satisfied, downstream-side NOx amount calculating means for integrating the downstream-side NOx flow amount to calculate the amount of NOx passing through the downstream side of the catalyst within a predetermined time when the condition concerned is determined to be satisfied, and trouble determining means for comparing the upstream-side NOx amount and the downstream-side NOx amount to determine whether the exhaust gas purification system operates normally.