Catalyst deterioration judging system

A catalyst deterioration judging method for judging deterioration of an absorption reduction type NOx catalyst which is provided at an exhaust gas passage of an internal combustion engine, the absorption reduction type NOx catalyst being configured to absorb NOx and to reduce the absorbed NOx based on contact with a reducing agent, the exhaust gas passage including at least one injection valve, the catalyst deterioration judging method comprising:

• supplying the reducing agent to the absorption reduction type NOx catalyst by supplying the reducing agent through the injection valve disposed in the engine exhaust passage such that an air-fuel ratio of an exhaust gas passing through the absorption reduction type NOx catalyst is changed;
• measuring a NOx concentration in the exhaust gas at a position downstream from the absorption reduction type NOx catalyst;
• regulating an amount of the reducing agent to be supplied so that the air-fuel ratio of the exhaust gas is a lean air-fuel ratio such that NH3 is not produced by the absorption reduction type NOx catalyst and NOx is released from the absorption reduction type NOx catalyst when judging the deterioration of the absorption reduction type NOx catalyst;
• and
• judging the deterioration of the absorption reduction type NOx catalyst based on the measured NOx concentration.