An exhaust gas purification system for an engine comprising a DPF for collecting PM in an exhaust gas exhausted from the engine to an exhaust passage and a PM accumulation state evaluation part for classifying a PM accumulation state of the DPF into multiple evaluation stages based on a plurality of evaluation indices, said PM accumulation state evaluation part including: a current stage determination part for determining a current evaluation stage; and an evaluation stage determination part for moving up the current evaluation stage to an evaluation stage of a next rank when a value of each of a prescribed number of the evaluation indices is greater than each threshold value; and being configured to repeatedly perform determination of the current evaluation stage by the current stage determination part and determination of whether to move up the current evaluation stage to the evaluation stage of the next rank by the evaluation stage determination part; characterized in that said PM accumulation evaluation part further includes: a defect detection part for detecting a defect of different types of sensors used for calculating each of the plurality of evaluation indices; and a current stage redetermination part for redetermining the current evaluation stage without using, among the different types of sensors, a sensor of which a defect is detected by the defect detection part; and being configured to newly redetermine the current evaluation stage by the current stage redetermination part as substituted for the current evaluation stage determined by the current stage determination part upon a defect of the sensor being detected by the defect detection part, by using an evaluation index other than an evaluation index based on an output value of the sensor of which a defect is detected, wherein upon a defect of a supply air flow meter as one of the sensors being detected by the defect detection part, an evaluation index is calculated based on a supply air flow rate calculated by an alternative unit as substituted for a supply air flow rate measured by the supply air flow meter of which a defect is detected, and the current evaluation stage is newly redetermined by the current stage redetermination part by using the evaluation index calculated by the alternative unit and at least one of the other evaluation indices. a current stage determination part for determining a current evaluation stage; and an evaluation stage determination part for moving up the current evaluation stage to an evaluation stage of a next rank when a value of each of a prescribed number of the evaluation indices is greater than each threshold value; and being configured to repeatedly perform determination of the current evaluation stage by the current stage determination part and determination of whether to move up the current evaluation stage to the evaluation stage of the next rank by the evaluation stage determination part; characterized in that said PM accumulation evaluation part further includes: a defect detection part for detecting a defect of different types of sensors used for calculating each of the plurality of evaluation indices; and a current stage redetermination part for redetermining the current evaluation stage without using, among the different types of sensors, a sensor of which a defect is detected by the defect detection part; and being configured to newly redetermine the current evaluation stage by the current stage redetermination part as substituted for the current evaluation stage determined by the current stage determination part upon a defect of the sensor being detected by the defect detection part, by using an evaluation index other than an evaluation index based on an output value of the sensor of which a defect is detected, wherein upon a defect of a supply air flow meter as one of the sensors being detected by the defect detection part, an evaluation index is calculated based on a supply air flow rate calculated by an alternative unit as substituted for a supply air flow rate measured by the supply air flow meter of which a defect is detected, and the current evaluation stage is newly redetermined by the current stage redetermination part by using the evaluation index calculated by the alternative unit and at least one of the other evaluation indices. a defect detection part for detecting a defect of different types of sensors used for calculating each of the plurality of evaluation indices; and a current stage redetermination part for redetermining the current evaluation stage without using, among the different types of sensors, a sensor of which a defect is detected by the defect detection part; and being configured to newly redetermine the current evaluation stage by the current stage redetermination part as substituted for the current evaluation stage determined by the current stage determination part upon a defect of the sensor being detected by the defect detection part, by using an evaluation index other than an evaluation index based on an output value of the sensor of which a defect is detected, wherein upon a defect of a supply air flow meter as one of the sensors being detected by the defect detection part, an evaluation index is calculated based on a supply air flow rate calculated by an alternative unit as substituted for a supply air flow rate measured by the supply air flow meter of which a defect is detected, and the current evaluation stage is newly redetermined by the current stage redetermination part by using the evaluation index calculated by the alternative unit and at least one of the other evaluation indices.