To obtain a heat medium boiler which performs pre-heating at a maximum pre-heating temperature according to the type of heat so that an NO_X concentration in an exhaust gas does not exceed a regulation value while maintaining high efficiency of a heat medium boiler.

The heat medium boiler includes: a recuperator 11 which pre-heats combustion air by performing heat-exchange between the exhaust gas emitted from the boiler for combusting gas fuel and the combustion air set tt a burner 2 using a blower 8; and a heat transfer area adjusting means 14 for adjusting a heat transfer are of the recuperator 11 according to the type of fuel.