A blood supply flow rate controlling device for use in an extracorporeal circulation device connected to a living body, the extracorporeal circulation device comprising a centrifugal pump for circulating blood through a blood circulation circuit, the blood supply flow rate controlling device comprising: rotational frequency detecting means that is configured to detect a rotational frequency of the centrifugal pump; inlet pressure detecting means and outlet pressure detecting means that is configured to detect an inlet pressure and an outlet pressure of the centrifugal pump, respectively; flow rate regulating means that is configured to regulate a flow rate of blood supplied to the living body by adjusting a flow path cross section area of the blood circulation circuit; and controlling means that is configured to control the flow rate regulating means, wherein the flow rate regulating means is located downstream of the centrifugal pump, the controlling means comprises an inputter, a storage, a processor, and an outputter, the inputter is configured to receive inputs of a desired target blood supply flow rate entered by an operator, an inlet pressure detection value and an outlet pressure detection value respectively detected by the inlet pressure detecting means and the outlet pressure detecting means, a detected rotational frequency of the centrifugal pump detected by the rotational frequency detecting means, and an estimated blood flow rate equation for calculating an estimated blood flow rate of blood passing through the blood circulation circuit from the inlet pressure detection value, the outlet pressure detection value, and the detected rotational frequency, the storage is configured to store the target blood supply flow rate, the inlet pressure detection value, the outlet pressure detection value, the detected rotational frequency, and the estimated blood flow rate equation, the processor is configured to calculate a differential pressure by subtracting the inlet pressure detection value from the outlet pressure detection value, is configured to calculate the estimated blood flow rate from the differential pressure and the detected rotational frequency by the estimated blood flow rate equation, and calculates a flow rate differential by subtracting the estimated blood flow rate from the target blood supply flow rate, and the outputter is configured to output to the flow rate regulating means a control signal that reduces the flow path cross section area when the flow rate differential has a negative value, is configured to output to the flow rate regulating means a control signal that increases the flow path cross section area when the flow rate differential has a positive value, and is configured to output to the flow rate regulating means a control signal that reduces the flow path cross section area to zero when the estimated blood flow rate has a negative value. rotational frequency detecting means that is configured to detect a rotational frequency of the centrifugal pump; inlet pressure detecting means and outlet pressure detecting means that is configured to detect an inlet pressure and an outlet pressure of the centrifugal pump, respectively; flow rate regulating means that is configured to regulate a flow rate of blood supplied to the living body by adjusting a flow path cross section area of the blood circulation circuit; and controlling means that is configured to control the flow rate regulating means, wherein