A deterioration diagnosis device, which performs a deterioration diagnosis of a catalyst, includes an exhaust-gas sensor provided downstream of the catalyst in a flow direction of exhaust gas such that an output value of the exhaust-gas sensor is used at least in the deterioration diagnosis. The deterioration diagnosis device further includes the constant current supply portion which applies a voltage to a sensor element of the exhaust-gas sensor to change an output characteristic of the exhaust-gas sensor, a response-time detection portion which detects a response time required for the output value of the exhaust-gas sensor to change from a rich threshold to a lean threshold, a response-time correction portion which controls the constant current supply portion to change the output characteristic of the exhaust-gas sensor so as to shorten the response time when the response time is longer than a predetermined reference time.