The present disclosure relates to ECU for an exhaust purging system comprises: a NOx catalyst provided in an exhaust passage; and a second composite sensor detecting an air-fuel ratio in a downstream of the NOx catalyst, the ECU, which performs a routine of a purge control, calculates the sum of values of the reductant that have been supplied to the NOx catalyst since the start of the routine of the purge control, determines whether the sum is greater than or equal to an end determination threshold, determines whether the air-fuel ratio is less than or equal to a predetermined value, and ends the routine of the purge control in response to an earlier one of a first affirmative determination that the sum is greater than or equal to the end determination threshold and a second affirmative determination that the air-fuel ratio is less than or equal to the predetermined value.