To determine whether an exhaust gas purification catalyst (42) of an engine (1) has deteriorated, first deterioration of the catalyst (42) is determined on the basis of an excess air ratio difference, which is a value expressing a difference in the oxygen concentration on the upstream side and downstream side of the catalyst (42). When the catalyst (42) is determined to have deteriorated on the basis of the excess air ratio difference, the air-fuel ratio of the engine (1) is feedback-controlled to a stoichiometric air-fuel ratio, and deterioration of the catalyst (42) is determined again on the basis of a value expressing a difference in the air-fuel ratio variation on the upstream side and downstream side of the catalyst (42) at that time.