System for and method of detecting deterioration of catalyst in internal combustion engine

An apparatus (41) for detecting deterioration of a catalyst (21) in an internal combustion engine (10) initially biases an air/fuel ratio of an air-fuel mixture supplied to the internal combustion engine (10) to a rich amount so that an amount of oxygen stored in the catalyst (21) is substantially zero, Then, the apparatus (41) detects deterioration of the catalyst (21) by alternating the air/fuel ratio lean or rich based on an amount of oxygen given to the catalyst (21). If the catalyst (21) has deteriorated, a bias amount of the air/fuel ratio is set so that the amount of oxygen stored in the catalyst (21) is substantially saturated. If the catalyst (21) is normal, a bias amount of the air/fuel ratio is set so that the amount of oxygen stored in the catalyst (21) is not saturated.