Method and apparatus for controlling an internal combustion engine

A control apparatus for an internal combustion engine having an oxygen storage capacity computing unit for computing an oxygen storage volume of a catalyst (120) using a real air / is detected by a front air / fuel ratio sensor for detecting exhaust gas components fuel ratio, wherein the front air / fuel ratio sensor on the upstream side of the catalyst (120) is installed, a corrected middle air / fuel ratio which is a stoichiometric air / fuel ratio of the catalyst (120), and an intake air volume, which is determined by a means for estimating or detecting a flow rate of air flowing into the catalyst (120), wherein said control means further comprises: a center air / fuel ratio correction means (207) which is adapted to the corrected built-in mid-air / fuel ratio based on the output of a on the downstream side of the catalyst (120) rear air / fuel ratio sensor, and to determine the oxygen storage by the volume calculating means (206) computed oxygen storage volume, wherein the center air / fuel ratio correction means (207) comprises a first catalyst air / fuel ratio estimating means (207A) for estimating an air / fuel ratio in the catalyst (120) based on the output of the rear air / fuel ratio sensor (126); a...