The combustor for treating exhaust gas has a flame holding portion (15) which is exposed to a combustion chamber (11), enclosed by a circumferential wall (13) and closed by a bottom wall (14). Combustion aid gas burner ports (23) through which a combustion aid gas (B) is ejected toward the flame holding portion (15) so as to generate a swirl are formed in the circumferential wall (13). Exhaust gas burner ports (22) for ejecting an exhaust gas (A) toward the flame holding portion (15) are formed in the bottom wall (14).
