To provide a multiple can installed boiler capable of enhancing efficiency of the whole of the boiler by reducing the frequency of start and stop of combustion and further, improving following properties to a load.

In the multiple can installed boiler, N units of boilers each of which has the setting of combustion stop, low combustion and high combustion are installed, such a combustion pattern that the number of units of low combustion or high combustion boilers is determined in accordance with the load is set, and the steam generation amount is controlled by changing the combustion amount of each boiler. Therein, N kinds of the combustion patterns from such a case that the number of combustion units is one to such a case that the number of combustion units is N are set, when being operated in a combustion pattern with a certain number of combustion units, the response to variation of the load is preferentially performed by changing the combustion amount of boiler burning without changing the number of combustion units of boilers, when the response to the variation of the load is disabled without changing the number of combustion units, the number of combustion units is changed, and when the increase of combustion amount results in only high combustion and combustion stop, control to increase the number of combustion units of boilers is performed.