A system (9) of the invention is a system for detecting deterioration of a catalyst (8) provided in an exhaust passage (12) of an internal combustion engine (11), the system including: means (5) that detects a temperature of an exhaust on an upstream side of the catalyst (8); means (6) that detects a temperature of the exhaust on a downstream side of the catalyst (8); a recognition unit (10) that recognizes, both upstream and downstream of the catalyst (8), a change point at which the temperature changes from temperature fall or constant temperature to temperature rise during acceleration operation, or a change point at which the temperature changes from temperature rise or constant temperature to temperature fall during deceleration operation; and a determination unit (10) that determines that the catalyst (8) has deteriorated when a difference (”T or ”T') between the upstream and downstream change points becomes equal to or longer than a predetermined time.
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