A catalyst deterioration detecting system for detecting deterioration of a catalyst provided in an exhaust passage of an internal combustion engine, the system comprising: an upstream-side temperature detection means that detects a temperature of an exhaust on an upstream side of the catalyst; a downstream-side temperature detection means that detects a temperature of the exhaust on a downstream side of the catalyst; and either a recognition unit that, during acceleration operation, recognizes a change point detected by the upstream-side temperature detection means at which the temperature changes from temperature fall or constant temperature to temperature rise and a change point detected by the downstream-side temperature detection means at which the temperature changes from temperature fall or constant temperature to temperature rise, and a determination unit that determines that the catalyst has deteriorated when a difference between the change points (ΔT=T2-T1) becomes equal to or longer than a predetermined time, or a recognition unit that, during deceleration operation, recognizes a change point (T1') detected by the upstream-side temperature detection means at which the temperature changes from temperature rise or constant temperature to temperature fall and a change point (T2') detected by the downstream-side temperature detection means at which the temperature changes from temperature rise or constant temperature to temperature fall, and a determination unit that determines that the catalyst has deteriorated when a difference between the change points (ΔT'=T2'-T1') becomes equal to or longer than a predetermined time. an upstream-side temperature detection means that detects a temperature of an exhaust on an upstream side of the catalyst; a downstream-side temperature detection means that detects a temperature of the exhaust on a downstream side of the catalyst; and either a recognition unit that, during acceleration operation, recognizes a change point detected by the upstream-side temperature detection means at which the temperature changes from temperature fall or constant temperature to temperature rise and a change point detected by the downstream-side temperature detection means at which the temperature changes from temperature fall or constant temperature to temperature rise, and a determination unit that determines that the catalyst has deteriorated when a difference between the change points (ΔT=T2-T1) becomes equal to or longer than a predetermined time, or a recognition unit that, during deceleration operation, recognizes a change point (T1') detected by the upstream-side temperature detection means at which the temperature changes from temperature rise or constant temperature to temperature fall and a change point (T2') detected by the downstream-side temperature detection means at which the temperature changes from temperature rise or constant temperature to temperature fall, and a determination unit that determines that the catalyst has deteriorated when a difference between the change points (ΔT'=T2'-T1') becomes equal to or longer than a predetermined time. a recognition unit that, during acceleration operation, recognizes a change point detected by the upstream-side temperature detection means at which the temperature changes from temperature fall or constant temperature to temperature rise and a change point detected by the downstream-side temperature detection means at which the temperature changes from temperature fall or constant temperature to temperature rise, and a determination unit that determines that the catalyst has deteriorated when a difference between the change points (ΔT=T2-T1) becomes equal to or longer than a predetermined time, or a recognition unit that, during deceleration operation, recognizes a change point (T1') detected by the upstream-side temperature detection means at which the temperature changes from temperature rise or constant temperature to temperature fall and a change point (T2') detected by the downstream-side temperature detection means at which the temperature changes from temperature rise or constant temperature to temperature fall, and a determination unit that determines that the catalyst has deteriorated when a difference between the change points (ΔT'=T2'-T1') becomes equal to or longer than a predetermined time.