An abnormality detecting apparatus for an EGR system of an internal combustion engine including a turbo charger having a turbine which is provided for an exhaust passage and a compressor which is provided for an intake passage, the EGR system of the internal combustion engine comprising: a high pressure EGR apparatus which has a high pressure EGR passage for connecting the exhaust passage upstream of the turbine and the intake passage downstream of the compressor, and a high pressure EGR valve for regulating a flow rate of a high pressure EGR gas introduced into the intake passage via the high pressure EGR passage; a low pressure EGR apparatus which has a low pressure EGR passage for connecting the exhaust passage downstream of the turbine and the intake passage upstream of the compressor, and a low pressure EGR valve for regulating a flow rate of a low pressure EGR gas introduced into the intake passage via the low pressure EGR passage; and control means which controls an EGR gas flow rate ratio (α) as a ratio between the flow rate of the high pressure EGR gas introduced into the intake passage and the flow rate of the low pressure EGR gas introduced into the intake passage so that the EGR gas flow rate ratio arrives at a prescribed target ratio, characterized in that the abnormality detecting apparatus for the EGR system comprises: estimating means which estimates the actual EGR gas flow rate ratio (α); allowable range calculating means which calculates a flow rate ratio allowable range as such a range of the EGR gas flow rate ratio (α) that a degree of deterioration of an exhaust characteristic of the internal combustion engine falls in an allowable level on the basis of an EGR ratio (Regr) as a ratio of an entire amount of the EGR gas with respect to an intake amount of the internal combustion engine by taking the target ratio as a reference; and judging means which judges that the EGR system is in an abnormality if the EGR gas flow rate ratio (α) estimated by the estimating means is deviated from the flow rate ratio allowable range calculated by the allowable range calculating means. a high pressure EGR apparatus which has a high pressure EGR passage for connecting the exhaust passage upstream of the turbine and the intake passage downstream of the compressor, and a high pressure EGR valve for regulating a flow rate of a high pressure EGR gas introduced into the intake passage via the high pressure EGR passage; a low pressure EGR apparatus which has a low pressure EGR passage for connecting the exhaust passage downstream of the turbine and the intake passage upstream of the compressor, and a low pressure EGR valve for regulating a flow rate of a low pressure EGR gas introduced into the intake passage via the low pressure EGR passage; and control means which controls an EGR gas flow rate ratio (α) as a ratio between the flow rate of the high pressure EGR gas introduced into the intake passage and the flow rate of the low pressure EGR gas introduced into the intake passage so that the EGR gas flow rate ratio arrives at a prescribed target ratio, characterized in that the abnormality detecting apparatus for the EGR system comprises: estimating means which estimates the actual EGR gas flow rate ratio (α); allowable range calculating means which calculates a flow rate ratio allowable range as such a range of the EGR gas flow rate ratio (α) that a degree of deterioration of an exhaust characteristic of the internal combustion engine falls in an allowable level on the basis of an EGR ratio (Regr) as a ratio of an entire amount of the EGR gas with respect to an intake amount of the internal combustion engine by taking the target ratio as a reference; and judging means which judges that the EGR system is in an abnormality if the EGR gas flow rate ratio (α) estimated by the estimating means is deviated from the flow rate ratio allowable range calculated by the allowable range calculating means. estimating means which estimates the actual EGR gas flow rate ratio (α); allowable range calculating means which calculates a flow rate ratio allowable range as such a range of the EGR gas flow rate ratio (α) that a degree of deterioration of an exhaust characteristic of the internal combustion engine falls in an allowable level on the basis of an EGR ratio (Regr) as a ratio of an entire amount of the EGR gas with respect to an intake amount of the internal combustion engine by taking the target ratio as a reference; and judging means which judges that the EGR system is in an abnormality if the EGR gas flow rate ratio (α) estimated by the estimating means is deviated from the flow rate ratio allowable range calculated by the allowable range calculating means.