PURPOSE:To always obtain highly accurate blood data without relying on the magnitude of velocity (v) by adjusting the magnitude of a sample volume by controlling a caliber or gate width on the basis of a cross-correlation function R(tau) so as to maximize the absolute value of saidfunction. CONSTITUTION:In an ultrasonic Doppler blood flowmeter obtaining blood flow data,an array probe is used and ultrasonic transmission-reception is performed three times at three different rates to obtain receiving echoes ei, ei+1, ei+2. From these echoes, signals vi, vi+2 (formulae II, III) after the removal of clutter determined on the basis of a range gate width are obtained. Subsequently, the cross-correlation coefficient R(tau) (formula III) between both signals vi, vi+1 is obtained and, from a value taumax of tau wherein R(tau) becomes max., blood flow velocity v=taumax.fr.c/2 (wherein fr is rate frequency and c is sonic velocity) is obtained. This flow velocity (v) is set to the mapping data on one ultrasonic raster and a series of these procedures are repeated with respect to each raster to form one frame. The caliber or range gate width of the array probe is controlled so as to make the absolute value of the aforementioned cross-correlation function R(tau) in this means max.