This disclosure provides a method for measuring an output characteristic of an air fuel ratio sensor based on an output value of the air fuel ratio sensor at the time of changing a fuel injection amount. Under a predetermined condition, an increase in the fuel injection amount is started from a particular cylinder and the increase in the fuel injection amount is continued for a given time period, or a decrease in the fuel injection amount is started from the particular cylinder and the decrease in the fuel injection amount is continued for the given time period. Therefore, the output characteristic of the air fuel ratio detecting module can be measured with sufficient accuracy, regardless of the contacting strength of emission gas for every cylinder against the air fuel ratio detecting module.