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 α, β, and K are calculated, whereby a cardiac output can be determined by an equation CO=K (α*PWTT+β)*HR.