To provide a catalyst deterioration detection device capable of accurately detecting degree of deterioration of a catalyst almost without deteriorating exhaust gas properties.

Quantity OS of oxygen flowing into a three-way catalyst 14a is calculated based on detected suction air flow rate GAIR and output of oxygen concentration sensor 17 provided in an upstream side of the three-way catalyst 14a. Rich/lean change over control (perturbation) of air fuel ratio is performed according to comparison results of quantity OS of flow in oxygen and target values OSOBJR, OSOBJL. When output SVO2 of an O2 sensor 18 provided in a downstream side fluctuates greatly during perturbation of air fuel ratio, it is judged that the three-way catalyst is deteriorated. O2 sensor output SVO2 has little fluctuation if the three-way catalyst 14a is normal by establishing the target values OSOBJR, OSOBJL appropriately.

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