An exhaust gas purification device downstream of a wet desulfurizer for a combustor has a vessel body for intake of exhaust gas from the wet desulfurizer at a lower portion and discharge of the same at an upper end, an auxiliary desulfurizing portion with an injection nozzles for injection of an alkaline solution at a position above an inlet in the vessel body to purify the exhaust gas, bag filters above the injection nozzles to separate particles in the exhaust gas, backwashing nozzles for injection of compressed gas from above the bag filters for backwashing, a drain-receiving tank for reception of drain from the vessel body and a supply pump for supply of the drain in the drain-receiving tank as alkaline solution to the injection nozzles.