A heat source device comprising: a first heat exchanger that cools or heats a heating medium that flows in from an external load; a second heat exchanger that performs heat exchange with external air or cooling water; a refrigerant circulating channel that circulates refrigerant between the first heat exchanger and the second heat exchanger; a centrifugal compressor provided in the refrigerant circulating channel and driven by an electrical motor whose rotational speed is controlled by an inverter; wherein the heat source device further comprises a differential-pressure measuring means for measuring a differential pressure between an inlet pressure and an outlet pressure of the heating medium in the first heat exchanger; and a controlling means configured to control the heat source device, wherein the controlling means includes a storage portion, a flow-rate computing means for calculating a flow rate (qa) of the heating medium in the first heat exchanger on the basis of the coefficient of loss (ζ) for the first heat exchanger stored in the storage portion and the differential pressure (dPe) output from the differential-pressure measuring means; a control-command computing means (55, 66) for generating a control command provided to each of control targets of the heat source device by using a specification heating-medium flow rate that is set in advance; and a control-command correcting means (56, 67) for correcting each of the control commands generated by the control-command computing means (55, 66) on the basis of the difference between the heating-medium flow rate calculated by the flow-rate computing means and the specification heating-medium flow rate, wherein the controlling means uses a control command corrected by the control-command correcting means (56, 67) to control the rotational speed of the electrical motor by means of the inverter, and wherein the flow-rate computing means possesses the following Equation and calculates the flow rate (qa) of the heating medium in the first heat exchanger by substituting the differential pressure (dPe) output from the differential-pressure measuring means into the Equation qa = ζ dPe. a first heat exchanger that cools or heats a heating medium that flows in from an external load; a second heat exchanger that performs heat exchange with external air or cooling water; a refrigerant circulating channel that circulates refrigerant between the first heat exchanger and the second heat exchanger; a centrifugal compressor provided in the refrigerant circulating channel and driven by an electrical motor whose rotational speed is controlled by an inverter; wherein the heat source device further comprises a differential-pressure measuring means for measuring a differential pressure between an inlet pressure and an outlet pressure of the heating medium in the first heat exchanger; and a controlling means configured to control the heat source device, wherein the controlling means includes a storage portion, a flow-rate computing means for calculating a flow rate (qa) of the heating medium in the first heat exchanger on the basis of the coefficient of loss (ζ) for the first heat exchanger stored in the storage portion and the differential pressure (dPe) output from the differential-pressure measuring means; a control-command computing means (55, 66) for generating a control command provided to each of control targets of the heat source device by using a specification heating-medium flow rate that is set in advance; and a control-command correcting means (56, 67) for correcting each of the control commands generated by the control-command computing means (55, 66) on the basis of the difference between the heating-medium flow rate calculated by the flow-rate computing means and the specification heating-medium flow rate, wherein the controlling means uses a control command corrected by the control-command correcting means (56, 67) to control the rotational speed of the electrical motor by means of the inverter, and wherein the flow-rate computing means possesses the following Equation and calculates the flow rate (qa) of the heating medium in the first heat exchanger by substituting the differential pressure (dPe) output from the differential-pressure measuring means into the Equation qa = ζ dPe.