An auxiliary heater control device comprising: a temperature sensor that senses a temperature of a fluid of a fluid circulation circuit for circulating the fluid, the fluid circulation circuit being configured by connecting, in order, a heat pump apparatus performing defrost operation in addition to normal operation and including a compressor that is controllable, an auxiliary heater that is controllable, a heated fluid utilization apparatus that utilizes the fluid heated by the normal operation of the heat pump apparatus, and a circulation pump that is controllable and circulates the fluid from the auxiliary heater to the heated fluid utilization apparatus, the temperature sensor sensing the temperature of the fluid flowing between the auxiliary heater and the heated fluid utilization apparatus; and a control unit that monitors a sensed temperature tS sensed by the temperature sensor during the defrost operation of the heat pump apparatus, and selectively controls at least one of the circulation pump, the compressor, and the auxiliary heater according to the sensed temperature tS; characterized in that the control unit controls at least either of the circulation pump and the compressor when the sensed temperature tS sensed by the temperature sensor is at or below a temperature tX and above a temperature tH, the temperature tX being a predetermined temperature at which to start control of either of the circulation pump and the compressor, and the temperature tH being a predetermined temperature which is lower than the temperature tX and at which to start control of the auxiliary heater, and wherein the control unit controls the circulation pump, the compressor, and the auxiliary heater when the sensed temperature tS is at or below the temperature tH. a temperature sensor that senses a temperature of a fluid of a fluid circulation circuit for circulating the fluid, the fluid circulation circuit being configured by connecting, in order, a heat pump apparatus performing defrost operation in addition to normal operation and including a compressor that is controllable, an auxiliary heater that is controllable, a heated fluid utilization apparatus that utilizes the fluid heated by the normal operation of the heat pump apparatus, and a circulation pump that is controllable and circulates the fluid from the auxiliary heater to the heated fluid utilization apparatus, the temperature sensor sensing the temperature of the fluid flowing between the auxiliary heater and the heated fluid utilization apparatus; and a control unit that monitors a sensed temperature tS sensed by the temperature sensor during the defrost operation of the heat pump apparatus, and selectively controls at least one of the circulation pump, the compressor, and the auxiliary heater according to the sensed temperature tS; characterized in that the control unit controls at least either of the circulation pump and the compressor when the sensed temperature tS sensed by the temperature sensor is at or below a temperature tX and above a temperature tH, the temperature tX being a predetermined temperature at which to start control of either of the circulation pump and the compressor, and the temperature tH being a predetermined temperature which is lower than the temperature tX and at which to start control of the auxiliary heater, and wherein the control unit controls the circulation pump, the compressor, and the auxiliary heater when the sensed temperature tS is at or below the temperature tH.