In an apparatus for diagnosing an exhaust gas purifying device, a catalyst deterioration level detector 104 detects the deterioration level of a catalyst from a relationship between a catalyst-upstream air-fuel-ratio detector 101 and a catalyst-downstream air-fuel-ratio detector 102. A catalyst-deterioration-level correction section 111 corrects the catalyst deterioration level detected by the catalyst deterioration level detector 104 based on a falling response ratio representing the ratio between a falling response index of the catalyst-upstream air-fuel-ratio detector 101 and a falling response index of the catalyst-downstream air-fuel-ratio detector 102 or based on a rising response ratio representing the ratio between a rising response index of the catalyst-upstream air-fuel-ratio detector 101 and a rising response index of the catalyst-downstream air-fuel-ratio detector 102. A catalyst deterioration judgment section 112 determines that the catalyst is deteriorated from information provided by the catalyst-deterioration-level correction section 111. This allows any deteriorated catalyst to be precisely detected even if an air-fuel ratio sensor downstream of the catalyst is degraded in terms of rising response.