PURPOSE: To enable keeping of the temperature of waste gas at a preferable catalyst temperature at the starting time of the titled device by a method wherein ducts bypassing a group of heat transmitting pipes positioned at an upstream of the denitrating device are arranged and the waste gas is passed through the bypass ducts at the time of starting.

CONSTITUTION: There are arranged a first bypassing duct 6 having a flow inlet port at the upstream of an over-heater unit 2 and also having an outlet port at the arranging part of the denitrating device, and a second bypassing duct 8 having a flow inlet port at the waste gas duct 1 opposing to the duct 6. When the boiler is started to operate, dampers 4 and 9 are closed and dampers 7 and 11 are opened, the waste gas G is flowed into the first bypassing duct 6 at the upstream of the over-heater unit 2, then bypasses the over-heater unit 2 and an evaporater unit 3, and directly flowed into the denitrating device 5. Therefore, the waste gas G is flowed into the denitrating device 5 without being removed of any heat, and thus it is possible to keep the catalyst at preferable temperature from the starting time.