A heating medium system comprising: a pump configured to circulate a heating medium in a circuit; a first heating device configured to heat the heating medium; a second heating device configured to heat the heating medium at a position downstream of the first heating device in a case where a heating power of the first heating device is insufficient; pump control means configured to control an operation of the pump such that a circulation flow rate of the heating medium or an operating speed of the pump matches a target value; and heating power control means configured to control the heating power of the first heating device such that a heating medium supply temperature, which is a temperature of the heating medium sent to a heat receiving section that receives heat of the heating medium, matches a target temperature; characterized in that the heating medium system further comprises prediction means configured to predict whether the second heating device will be activated, wherein the pump control means is configured to: control the operation of the pump such that the circulation flow rate of the heating medium or the operating speed of the pump matches a first target value in a case where it is predicted that the second heating device will not be activated; and control the operation of the pump such that the circulation flow rate of the heating medium or the operating speed of the pump matches a second target value larger than the first target value in a case where it is predicted that the second heating device will be activated. a pump configured to circulate a heating medium in a circuit; a first heating device configured to heat the heating medium; a second heating device configured to heat the heating medium at a position downstream of the first heating device in a case where a heating power of the first heating device is insufficient; pump control means configured to control an operation of the pump such that a circulation flow rate of the heating medium or an operating speed of the pump matches a target value; and heating power control means configured to control the heating power of the first heating device such that a heating medium supply temperature, which is a temperature of the heating medium sent to a heat receiving section that receives heat of the heating medium, matches a target temperature; characterized in that the heating medium system further comprises prediction means configured to predict whether the second heating device will be activated, wherein the pump control means is configured to: control the operation of the pump such that the circulation flow rate of the heating medium or the operating speed of the pump matches a first target value in a case where it is predicted that the second heating device will not be activated; and control the operation of the pump such that the circulation flow rate of the heating medium or the operating speed of the pump matches a second target value larger than the first target value in a case where it is predicted that the second heating device will be activated. control the operation of the pump such that the circulation flow rate of the heating medium or the operating speed of the pump matches a first target value in a case where it is predicted that the second heating device will not be activated; and control the operation of the pump such that the circulation flow rate of the heating medium or the operating speed of the pump matches a second target value larger than the first target value in a case where it is predicted that the second heating device will be activated.