A mercury removing device includes a gasification unit that converts a non-gaseous-mercury-chlorinating agent, which is non-gaseous at room temperature and normal pressure, into gaseous-mercury-chlorinating agent by heating the non-gaseous-mercury-chlorinating agent with heat of hot air generated by using the exhaust gas or hot air generated by using an air heater installed in a flue that conveys the exhaust gas. The gaseous-mercury-chlorinating agent produced in this manner is supplied to the exhaust gas in the flue.