To improve the follow-up property of a boiler to load changes and, at the same time, to suppress an increase in the number of starting and stopping times of the boiler.

For the boiler 1 the combustion amount of which is controlled in multiple stages, the gradient of the detected value of the steam pressure, hot water delivery temperature, etc., used for controlling combustion is detected and a combustion amount switching instructing signal is outputted based on the detected gradient during combustion.