A combustion control method during a rich spike engine operation includes the steps of: specifying an exhaust gas A/F based on a first output from a NOx sensor on an upstream side of a catalyst; specifying a NOx concentration in the exhaust gas on the upstream side of the catalyst based on a second output from the NOx sensor; and when the A/F is less than a reference range and the NOx concentration is not more than a reference value, calculating a value P1−P0 between a cylinder pressure P1 at EVO during the rich spike operation and that pressure P0 during a normal operation, when P1−P0 is larger than the reference range and P1>P0, a fuel injection timing at a post injection is retarded, when P1−P0 is smaller than the reference range and P1