A catalyst-degradation detection device, comprising: a three-way catalyst that is disposed in an exhaust passage of an internal combustion engine to purify exhaust emission; air-fuel ratio active control means that executes air-fuel ratio active control that forcefully and periodically switches a target air-fuel ratio at an upstream side from the three-way catalyst to a richer side and a leaner side than stoichiometry; a downstream side air-fuel ratio sensor that detects an exhaust air-fuel ratio at a downstream side from the three-way catalyst; and oxygen storage capacity degradation determining means that determines degradation relating to an oxygen storage capacity of the three-way catalyst by using the exhaust air-fuel ratio detected by the downstream side air-fuel ratio sensor during execution of the air-fuel ratio active control, an upstream side air-fuel ratio sensor that detects an exhaust air-fuel ratio at an upstream side from the three-way catalyst; and exhaust emission purification capacity determining means that determines degradation relating to an exhaust emission purification capacity of the three-way catalyst by using the exhaust air-fuel ratios that are detected by the upstream side air-fuel ratio sensor and the downstream side air-fuel ratio sensor during execution of the air-fuel ratio active control, wherein the oxygen storage capacity degradation determining means determines degradation relating to the oxygen storage capacity of the three-way catalyst by using an oxygen storage amount that is calculated by using an exhaust air-fuel ratio detected by the downstream side air-fuel ratio sensor in a storage time period from a predetermined starting point in a rich region until a predetermined end point after reaching stoichiometry and before shifting to a lean region at a time of switching the target air-fuel ratio from rich to lean, or an oxygen emission amount that is calculated by using an exhaust air-fuel ratio detected by the downstream side air-fuel ratio sensor in an emission time period from a predetermined starting point in the lean region until a predetermined end point after reaching stoichiometry and before shifting to the rich region at a time of switching the target air-fuel ratio from lean to rich, and wherein the exhaust emission purification capacity determining means determines degradation relating to the exhaust emission purification capacity of the three-way catalyst by using an oxygen storage change speed that is calculated by using the exhaust air-fuel ratios that are detected by the upstream side air-fuel ratio sensor and the downstream side air-fuel ratio sensor in the storage time period, or an oxygen emission change speed that is calculated by using the exhaust air-fuel ratios detected by the upstream side air-fuel ratio sensor and the downstream side air-fuel ratio sensor in the emission time period. a three-way catalyst that is disposed in an exhaust passage of an internal combustion engine to purify exhaust emission; air-fuel ratio active control means that executes air-fuel ratio active control that forcefully and periodically switches a target air-fuel ratio at an upstream side from the three-way catalyst to a richer side and a leaner side than stoichiometry; a downstream side air-fuel ratio sensor that detects an exhaust air-fuel ratio at a downstream side from the three-way catalyst; and oxygen storage capacity degradation determining means that determines degradation relating to an oxygen storage capacity of the three-way catalyst by using the exhaust air-fuel ratio detected by the downstream side air-fuel ratio sensor during execution of the air-fuel ratio active control,