To prevent an excess rise of a steam temperature by correcting a boiler load based on a target mixed fuel combustion ratio of oil and coal at the time of completing starting of a coal fired boiler at the time of the mixed fuel combustion in the boiler for sequentially starting a plurality of mills, and sequentially starting the residual mills at each time of arriving the correcting load at the starting load.

At the time of starting the coal fired boiler, a flow control valve 7 is regulated, a predetermined amount of fuel and oil is injected from a desired fuel/oil burner 3 and burned. When a boiler load becomes larger than a first starting load preset to a signal monitoring switch, a first start command 9 is outputted to a desired mill 5. When the boiler load becomes larger than a second starting load, a second start command 9 is outputted. However, when the boiler load becomes larger than a third starting load, a correction coefficient obtained based on a target mixed fuel combustion ratio of oil coal at the time of completing starting of the boiler is multiplied by the boiler load, corrected, and, when the boiler load after the correction becomes larger than a third starting load, a third starting command 9 is outputted.