A processing device included in a cardiac output monitoring system includes an arterial diameter measurement ultrasonic probe that is attached to a neck of a subject, and measures the diameter of the carotid artery, and an arterial diameter measurement control section. A carotid artery diameter control section analyzes an ultrasonic echo to calculate the diameter of the artery (DOA), and outputs the diameter of the artery (DOA) to an ejection time/mean arterial pressure calculation section. The ejection time/mean arterial pressure calculation section calculates the ejection time (ET) from a temporal change in the diameter of the artery (DOA), and calculates the cardiac output. The temporal change waveform of the diameter of the artery (DOA) is clearer than the waveform of the blood flow velocity in the aorta or the ventricular outflow tract measured using an ultrasonic Doppler method, and the ejection time (ET) can be automatically calculated with high accuracy.
