Whether discharge of high-concentration, unburned fuel is indicated or not is determined based on air-fuel ratio AF, calculated air-fuel ratio AFc and exhaust temperature thco (S110, S112, S114). If it is determined that high-concentration, unburned fuel would be discharged ("no" in any of S110, S112, S114), amount of fuel addition per one addition is decreased in PM regeneration control (S116). Therefore, in a state where high concentration HC is to be discharged to the downstream of an exhaust purifying filter, the amount of fuel addition at one time is immediately decreased, so that discharge of high-concentration HC to the outside can be prevented, and generation of white smoke can reliably be prevented.