To shorten the burn up time for pulverized coal and make a heat recovery device compact.

A combustor comprises at least one pair of burners 3a, 3b disposed on a furnace wall 1a of a combustion furnace 1A, a pulverized coal supply pipe 14 connected to an inner cylinder 3B of the burner, a pipe 15 for supplying combustion air and discharging combustion exhaust gas whose one end is connected to a path 3A doubling as a combustion air supplying path and a combustion exhaust gas discharging path, with the other end of the pipe 15 being connected to a four-way switching valve 17, and a cyclone 20 and a heat exchanger 16 which are intermediately connected between the burners 3a, 3b and the valve 17. By periodically switching the valve 17, heated combustion air 11 is transferred to one burner 3a through the heat exchanger 16, and thus pulverized coal 19 transferred from the cylinder 3B is combusted. Further, combustion exhaust gas 13 which has flowed back through the other burner 3b is transferred to the heat exchanger 16 through the cyclone 20 to conduct cooling of the gas 13 as well as heating of the heat exchanger 16, and thus fly 21 ash contained in the gas 13 is removed by the cyclone 20.