Method for measuring a blood volume is provided. At least two types of respiratory variation data； for instance, data pertinent to respiratory variations in stroke volume (SVV) data, data pertinent to respiratory variations in an amplitude of a pulse wave (PAV), a pulse wave transit time (PWTT) in a respiratory cycle, and a heart rate (HR) in a predetermined time, are measured, and patient's inherent coefficients ±, ², and K are calculated, whereby a cardiac output can be determined by an equation CO = K (± * PWTT + ²) * HR.
