PURPOSE: To observe a low flow velocity blood flow without deteriorating the maximum visual field depth, the number of frames and the picture quality by making a sampling frequency of Doppler information lower than an ultrasonic pulse repeat frequency, and also, maintaining a data collection interval so as to be constant.

CONSTITUTION: An ultrasonic probe 11 transmits and receives an ultrasonic pulse to and from a subject. Also, a sector electronic scanner analog part 12 is constituted of a pre-amplifier 13, an oscillator 14, a delay line 16, an adder 17 and a detector 18, and changes and controls ultrasonic scanning order by a control circuit 35. Moreover, an MTI arithmetic part 27 is constituted of an A/D converter 28, plural line memories 34 and MTI filters 29, a self- correlator 30, an average speed arithmetic part 31, a distributed arithmetic part 32 and a power arithmetic part 33. In such a state, each arithmetic value is inputted to a D.S.C. 19, and converted to color information by a color processing circuit 20. In this case, a sampling frequency is made lower than an ultrasonic pulse repeat frequency, and also, a data collection interval is maintained at a constant level.