Exhaust gas purification apparatus for an internal combustion engine

An exhaust gas purification apparatus for an internal combustion engine comprising:

• a first catalyst in a form of a three-way catalyst that is arranged in an exhaust passage of the internal combustion engine, and has an oxygen storage capacity;
• a second catalyst in the form of a three-way catalyst that is arranged in said exhaust passage at a downstream side of said first catalyst, and has an oxygen storage capacity;
• and
• a controller configured to make a target air fuel ratio of said internal combustion engine vary;
• wherein said controller configured to carry out lean control and rich control in an alternate manner, said lean control setting a reference lean air fuel ratio, which is an air fuel ratio set as a target mean value of the target air fuel ratio at the time of making the target air fuel ratio of said internal combustion engine vary and which is an air fuel ratio leaner than a stoichiometric air fuel ratio, said lean control making the target air fuel ratio of said internal combustion engine vary between a first lean air fuel ratio, which is an air fuel ratio leaner than said reference lean air fuel ratio, and a first rich air fuel ratio, which is an air fuel ratio richer than the stoichiometric air fuel ratio, and said rich control setting a reference rich air fuel ratio, which is an air fuel ratio set as a target mean value of the target air fuel ratio at the time of making the target air fuel ratio of said internal combustion engine vary and which is an air fuel ratio richer than the stoichiometric air fuel ratio, said rich control making the target air fuel ratio of said internal combustion engine vary between a second lean air fuel ratio, which is an air fuel ratio leaner than the stoichiometric air fuel ratio, and a second rich air fuel ratio, which is an air fuel ratio richer than the reference rich air fuel ratio;
• and
• when making said target air fuel ratio vary in the case of carrying out said lean control, said controller configured to make makes a period per one time in which said target air fuel ratio becomes said first lean air fuel ratio longer than a period per one time in which said target air fuel ratio becomes said first rich air fuel ratio, and make a deviation of said first lean air fuel ratio from said reference lean air fuel ratio smaller than a deviation of said first rich air fuel ratio from said reference lean air fuel ratio.