A heat source system comprising: heat source means for producing a heating medium at a desired temperature and configured to supply the heating medium to a load; a first pipe configured to supply the heating medium from the heat source means to the load; a supply header which is provided in the first pipe; a second pipe configured to supply the heating medium used by the load to the heat source means; a return header which is provided in the second pipe; a bypass pipe for bypassing the heating medium from the supply header to the return header; a bypass valve which is provided in the bypass pipe and for adjusting the bypass flow rate of the heating medium; a pump which is provided on an upstream side of a heating medium flow from the supply header in the first pipe or on a downstream side of a heating medium flow from the return header in the second pipe and has a variable rotational speed; and control means for adjusting the opening of the bypass valve, wherein the control means includes opening command value determination means configured for determining a bypass valve opening command value such that a header differential pressure (ΔP) which is the differential pressure between the supply header and the return header matches a target differential pressure value, target opening value setting means configured for setting a target opening value to an initial value and configured for changing the target opening value according to the header differential pressure or the behavior of the bypass valve opening, and heating medium flow rate setting means configured for holding an opening-flow rate characteristic in which the relationship between a bypass valve opening command value and a heating medium flow rate set value is defined in association with a target opening value, and configured for acquiring the heating medium flow rate set value corresponding to the valve opening command value determined by the opening command value determination means using the opening-flow rate characteristic corresponding to the target opening value set by the target opening value setting means, and the control means is configured so that a frequency control of the pump is performed so that the total flow rate fed from the pump becomes the heating medium flow rate set value. heat source means for producing a heating medium at a desired temperature and configured to supply the heating medium to a load; a first pipe configured to supply the heating medium from the heat source means to the load; a supply header which is provided in the first pipe; a second pipe configured to supply the heating medium used by the load to the heat source means; a return header which is provided in the second pipe; a bypass pipe for bypassing the heating medium from the supply header to the return header; a bypass valve which is provided in the bypass pipe and for adjusting the bypass flow rate of the heating medium; a pump which is provided on an upstream side of a heating medium flow from the supply header in the first pipe or on a downstream side of a heating medium flow from the return header in the second pipe and has a variable rotational speed; and control means for adjusting the opening of the bypass valve, wherein the control means includes opening command value determination means configured for determining a bypass valve opening command value such that a header differential pressure (ΔP) which is the differential pressure between the supply header and the return header matches a target differential pressure value, target opening value setting means configured for setting a target opening value to an initial value and configured for changing the target opening value according to the header differential pressure or the behavior of the bypass valve opening, and heating medium flow rate setting means configured for holding an opening-flow rate characteristic in which the relationship between a bypass valve opening command value and a heating medium flow rate set value is defined in association with a target opening value, and configured for acquiring the heating medium flow rate set value corresponding to the valve opening command value determined by the opening command value determination means using the opening-flow rate characteristic corresponding to the target opening value set by the target opening value setting means, and the control means is configured so that a frequency control of the pump is performed so that the total flow rate fed from the pump becomes the heating medium flow rate set value.