A B-mode Doppler flow mapping apparatus includes a transducer for radiating an ultrasonic wave to an object, and receiving an ultrasonic wave reflected by the object, a scanning circuit for controlling the transducer to change a radiation direction of the ultra-sonic wave in such a manner that a given slice of the object is scanned by the ultrasonic waves while the ultrasonic waves are repeatedly radiated n (n.gtoreq.2) number of times, each repetition being after the completion of a given number of radiations, a B-mode processor for obtaining a tomographic image of the slice by detecting the intensity of the ultrasonic waves received, and a color mapping processor for detecting a Doppler shift frequency of the ultrasonic wave received, and performing a color display of a blood flow in the tomographic image in accordance with the Doppler shift frequency. The apparatus includes a manual operating member for a designating an approximate flow rate of a blood flow in the slice, and a control circuit for controlling the scanning circuit in accordance with the flow rate designated by the manual operating member and changing the given number of radiations so as to change a sampling frequency for detecting a Doppler shift frequency, and to set optimal upper and lower limits of detectable Doppler shift frequencies.