an exhaust control system for an internal combustion engine provided with an exhaust gas processing device and a nox purifying catalyst which are arranged in series in an exhaust system , comprises : a first temperature detector for detecting a temperature of the exhaust gas processing device ; a second temperature detector for detecting a temperature of the nox purifying catalyst ; a control means for controlling an exhaust temperature to conduct a regeneration process for removing sulfur contents trapped by the nox purifying catalyst ; and a control mode selection means for selecting one of a plurality of exhaust temperature control modes according to a relationship between an output from the first temperature detector and an output from the second temperature detector , wherein the control means conducts exhaust temperature control according to the control mode selected by the control mode selection means .