During execution of a first purification process of fluctuating a hydrocarbon concentration in exhaust gas flowing into a first catalyst with an amplitude within a prescribed range at a time interval within a prescribed range, when a switch request to a second purification process of purifying NOx in a second catalyst by adding urea water into the exhaust gas is generated, the switch to the second purification process is prohibited on the condition that a current NOx purification rate (a first purification rate R1) is higher than a purification rate (a second purification rate R2) on the assumption that the second purification process is executed, and an HC poisoning recovery stand-by process of reducing an additive amount of hydrocarbon per once in the first purification process is executed so as to reduce a slip amount of hydrocarbon into the downstream of the first catalyst.