A device includes: downstream air-fuel ratio feedback control unit that performs feedback control of an upstream target air-fuel ratio so that a downstream air-fuel ratio detected by a downstream air-fuel ratio detecting unit coincides with a downstream target air-fuel ratio; a target air-fuel ratio varying unit that varies the upstream target air-fuel ratio at the time of diagnosing degradation of a ternary catalyst; and a catalyst degradation judging unit that judges catalyst degradation from the behavior of the downstream air-fuel ratio at the time of diagnosing degradation of the ternary catalyst. When diagnosing degradation of the ternary catalyst, the downstream air-fuel ratio feedback control unit is stopped or its control constant is set at a smaller value. Thus, when varying the air-fuel ratio for diagnosing catalyst degradation, interference between downstream air-fuel ratio feedback and upstream air-fuel ratio feedback is prevented and catalyst degradation is accurately diagnosed.