A boiler system equipped with a boiler group mixedly provided with a step value control boiler and a proportional control boiler. A boiler number control device is configured to control the number of boilers in the boiler group, and includes an output controller configured to control a combustion state of the boiler group so as to cause the proportional control boiler to output steam equivalent to a required steam flow according to a required load, and an output switcher configured to switch, under a condition that a steam flow outputted from the proportional control boiler reaches a predetermined steam flow exceeding a steam flow at a possible combustion point of the step value control boiler, output of the steam flow at the combustion point from the proportional control boiler to the step value control boiler.