To perform catalyst degradation determination while suppressing deterioration of emission.
The degradation determining device (1) for an air-fuel ratio sensor is provided with: an operating state detecting means (31) for detecting an operating state of an internal combustion engine (10); a control means (31) for controlling a fuel supply means (15) so that a first air-fuel ratio in an exhaust passage (14) approaches stoichiometry and second air-fuel ratios in each of at least two cylinders out of a plurality of cylinders (12a-12d) differ within a first range, under the condition that the detected operating state of the internal combustion engine (10) is in a predetermined state, and thereafter, controlling the fuel supply means (15) so that the first air-fuel ratio approaches stoichiometry and the second fuel ratios differ within a second range; and a determining means (31) for determining whether the air-fuel ratio sensor is deteriorated, based on a first output which is an output of the air fuel sensor when the second air-fuel ratios differ within the first range, and a second output which is an output of the air-fuel ratio sensor when the second air-fuel ratios differ within the second range.
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Koji Yamamoto
Satoshi Nakamura