An ultrasound diagnostic apparatus comprising: a transducer array that acquires a reception signal by transmitting and receiving an ultrasound wave to and from a subject; a B-mode processing unit that generates a B-mode image in which at least a blood vessel is captured based on the reception signal; a display device that displays the B-mode image generated by the B-mode processing unit; a first vascular wall detection unit that detects a vascular wall in a minor axis direction by analyzing the B-mode image in which a minor axis image of the blood vessel is captured; a first blood vessel diameter calculation unit that calculates a first blood vessel diameter based on the vascular wall in the minor axis direction detected by the first vascular wall detection unit; a second vascular wall detection unit that detects a vascular wall in a major axis direction by analyzing the B-mode image in which a major axis image of the blood vessel is captured; a second blood vessel diameter calculation unit that calculates a second blood vessel diameter based on the vascular wall in the major axis direction detected by the second vascular wall detection unit; a gate setting unit that sets a Doppler gate in the blood vessel on the B-mode image in which the major axis image is captured; a Doppler processing unit that acquires Doppler data in the Doppler gate; a blood flow velocity calculation unit that calculates a blood flow velocity based on the Doppler data; and a blood flow rate measurement unit that measures a blood flow rate based on any one of the detected vascular wall in the major axis direction or the detected vascular wall in the minor axis direction and the calculated blood flow velocity, wherein in a case where the second blood vessel diameter calculated by the second blood vessel diameter calculation unit is within a determined range with respect to the first blood vessel diameter calculated by the first blood vessel diameter calculation unit, the blood flow rate is automatically measured. a transducer array that acquires a reception signal by transmitting and receiving an ultrasound wave to and from a subject; a B-mode processing unit that generates a B-mode image in which at least a blood vessel is captured based on the reception signal; a display device that displays the B-mode image generated by the B-mode processing unit; a first vascular wall detection unit that detects a vascular wall in a minor axis direction by analyzing the B-mode image in which a minor axis image of the blood vessel is captured; a first blood vessel diameter calculation unit that calculates a first blood vessel diameter based on the vascular wall in the minor axis direction detected by the first vascular wall detection unit; a second vascular wall detection unit that detects a vascular wall in a major axis direction by analyzing the B-mode image in which a major axis image of the blood vessel is captured; a second blood vessel diameter calculation unit that calculates a second blood vessel diameter based on the vascular wall in the major axis direction detected by the second vascular wall detection unit; a gate setting unit that sets a Doppler gate in the blood vessel on the B-mode image in which the major axis image is captured; a Doppler processing unit that acquires Doppler data in the Doppler gate; a blood flow velocity calculation unit that calculates a blood flow velocity based on the Doppler data; and a blood flow rate measurement unit that measures a blood flow rate based on any one of the detected vascular wall in the major axis direction or the detected vascular wall in the minor axis direction and the calculated blood flow velocity, wherein in a case where the second blood vessel diameter calculated by the second blood vessel diameter calculation unit is within a determined range with respect to the first blood vessel diameter calculated by the first blood vessel diameter calculation unit, the blood flow rate is automatically measured.