An exhaust purification system comprising: a NOx reduction catalyst which is provided on an exhaust passage of an internal combustion engine to reduce and purify NOx contained in exhaust gas which flows inside the exhaust passage; an injection system (11, 34) configured to inject a fuel; a filter which is provided on the exhaust passage to collect particulate matters in the exhaust gas; catalyst regeneration means for executing a catalyst regeneration control for restoring the NOx reduction catalyst from sulfur poisoning by increasing a temperature of the exhaust gas to a predetermined first target temperature at which sulfur oxides contained in the catalyst are desorbed through an injection system control which controls the injection system (11, 34) to increase at least a fuel injection amount of the fuel; prohibition means for prohibiting an execution of the catalyst regeneration control according to an operating state of the internal combustion engine; temperature retention control means for executing a catalyst temperature retention control for maintaining the temperature of the exhaust gas at a predetermined second target temperature which is lower than the first target temperature by controlling the fuel injection amount during a period of time during which the execution of the catalyst regeneration control is prohibited by the prohibition means; and filter regeneration means for executing a filter regeneration control in which the temperature of the exhaust gas is raised to a combustion temperature of the particulate matters collected on the filter by increasing the fuel injection amount; wherein the prohibition means is configured to prohibit the execution of the catalyst regeneration control when a catalyst temperature of the NOx reduction catalyst is decreased below the second target temperature; and wherein the catalyst regeneration means is configured to start the catalyst regeneration control when the filter regeneration means ends the filter regeneration control, and the second target temperature which is used for the catalyst temperature retention control is set at the combustion temperature of the particulate matters. a NOx reduction catalyst which is provided on an exhaust passage of an internal combustion engine to reduce and purify NOx contained in exhaust gas which flows inside the exhaust passage; an injection system (11, 34) configured to inject a fuel; a filter which is provided on the exhaust passage to collect particulate matters in the exhaust gas; catalyst regeneration means for executing a catalyst regeneration control for restoring the NOx reduction catalyst from sulfur poisoning by increasing a temperature of the exhaust gas to a predetermined first target temperature at which sulfur oxides contained in the catalyst are desorbed through an injection system control which controls the injection system (11, 34) to increase at least a fuel injection amount of the fuel; prohibition means for prohibiting an execution of the catalyst regeneration control according to an operating state of the internal combustion engine; temperature retention control means for executing a catalyst temperature retention control for maintaining the temperature of the exhaust gas at a predetermined second target temperature which is lower than the first target temperature by controlling the fuel injection amount during a period of time during which the execution of the catalyst regeneration control is prohibited by the prohibition means; and filter regeneration means for executing a filter regeneration control in which the temperature of the exhaust gas is raised to a combustion temperature of the particulate matters collected on the filter by increasing the fuel injection amount; wherein the prohibition means is configured to prohibit the execution of the catalyst regeneration control when a catalyst temperature of the NOx reduction catalyst is decreased below the second target temperature; and wherein the catalyst regeneration means is configured to start the catalyst regeneration control when the filter regeneration means ends the filter regeneration control, and the second target temperature which is used for the catalyst temperature retention control is set at the combustion temperature of the particulate matters.