It is possible to realize flow rate control regardless of the scale on a load side or a piping system and to achieve energy saving. In a host control device (20) of a heat source system, a bypass valve opening command value is determined by an opening command value determination unit (22) such that a header differential pressure matches a target differential pressure value, and a target opening value according to the header differential pressure or the behavior of the bypass valve opening is set by a target opening value setting unit (24). A heating medium flow rate set value is determined by a heating medium flow rate setting unit (23) using the target opening value set by the target opening value setting unit (24) and the opening command value determined by the opening command value determination unit (22).