An electronic control device calculates the maximum actual oxygen storage capacity of a catalyst. The gradient of a linear expression formed between the catalyst temperature and the maximum oxygen storage capacity of the catalyst is stored for each degradation level of the catalyst. The gradient can be learned in accordance with the same temperature of the catalyst and the maximum actual oxygen storage capacity. When the maximum actual oxygen storage capacity is calculated, it is revised in accordance with the temperature of the catalyst, a reference temperature, the linear expression, and the learned gradient. The revised maximum oxygen storage capacity which is the maximum oxygen storage capacity when the temperature of the catalyst during the same calculation period is equal to the reference temperature is then calculated. If a response delay is detected in the output of an oxygen sensor, the gradient of the learned linear expression is discarded.