Disclosed is a combustion system that enables reduction in nitrogen oxide discharged from exhaust gas. The combustion system comprises: a combustion furnace (2) which has a burner unit (2a) that supplies fuel and oxygen for combustion to inside a furnace, a reduction region formed on the downstream side of the burner unit (2a) and where fuel is combusted, and a combustion oxygen supply port (2b) which supplies the oxygen for combustion (21) so that unburned fuel that has passed through the reduction region is completely combusted; and a smoke removal device (9) that removes smoke from the exhaust gas that is discharged from the combustion furnace (2). The combustion system is characterized by part of the exhaust gas (22) diverted from between the combustion furnace (2) and the smoke removal device (9) being guided to the burner unit (2a) and part of the exhaust gas (23) diverted from the downstream side of the smoke removal device (9) being guided to the combustion oxygen supply port (2b).