PROBLEM TO BE SOLVED: To provide a flow sensor which can accurately measure the flow rate value of blood in correspondence with the change in characteristics of the blood of a patient changing momentarily and with adaptation to a tube or the like used at that time in a case where a pump is operated to perform blood circulation, and provide an extracorporeal circulation apparatus using the flow sensor and a control method thereof.SOLUTION: An extracorporeal circulation apparatus for using a pump arranged in a circulation circuit to circulate the blood of a patient outside the body of the patient comprises: a flow sensor 30 which is arranged in the circulation circuit to obtain the measurement flow rate value of the blood passing through the circulation circuit by using an ultrasonic signal ultrasonic transmission units 32, 33, 34, 35 which are provided so as to face each other at deviated positions with the circulation circuit held therebetween and an ultrasonic reception unit which receives the ultrasonic waves transmitted from the ultrasonic transmission units an angle change unit 36 which changes the transmission angle of the ultrasonic wave of the ultrasonic transmission unit and a control part 100.
