An internal combustion engine control device for, in a lean-burn internal combustion engine provided with a three-way catalyst and a lean NOx catalyst in an exhaust system, optimizing an air/fuel ratio in a rich mode from the viewpoints of both better fuel economy and exhaust emission control when NOx having been stored in a lean NOx catalyst during a lean operation is desorbed and cleaned by switching to a rich operation.An internal combustion engine control device for controlling an air/fuel ratio during the rich operation to be changed between a period of desorbing O.sub.2 stored in the three-way catalyst and a period of desorbing and cleaning NOx stored in the lean NOx catalyst.