To suppress unevenness in discharge of an injurious ingredient to a low state through simple system, to suppress sudden discharge of CO and HC even under a transient operation condition even under transient operation, to reduce the size and a cost of a three-dimensional catalyst, and to accurately detect deterioration of the three-dimensional catalyst.

A control method for an engine exhaust gas system is executed such that the air-fuel ratio of exhaust gas discharged from an engine 30 is detected by an oxygen sensor 71, the air-fuel ratio is controlled to a value approximately equal to a theoretical value through a closed loop control using a signal therefrom and exhaust gas is guided to three-dimensional catalyst 40 to treat NOX and I-H and COX. An NOX sensor 70 is arranged in the downstream spot of the three-dimensional catalyst 40, an air-fuel ratio control value through closed loop control by the oxygen sensor 71 is corrected by the output of the NOX sensor 70, and NX concentration is adjusted to a given value. Further, based on the correction value of a current air-fuel ratio control value or the change width thereof, deterioration of the three-dimensional catalyst 40 and/or oxygen sensor 71 is detected.