To provide a troubleshooting device of an exhaust gas recirculation system of an internal combustion engine capable of increasing a trouble detection accuracy in consideration of the parameters of disturbance, i.e., the effect of variation due to those other than troubles of the internal combustion engine.

A diesel engine 11 comprises an EGR device 26. An ECU 13 comprises a microcomputer 45 and a memory 46, sets a target EGR rate based on the operating state of the diesel engine 11 and estimates an actual EGR rate, and calculates a feedback amount for changing the opening of an EGR valve so that a difference between both rates can be reduced. The ECU 13 adds the feedback amount against the target EGR rate to a learned value under the conditions that learning conditions under which the effect of the cause of disturbance becomes a specified value are satisfied for updating. The ECU 13 determines that the engine is defective when the updated learned value does not come within a specified set range.