The present invention discloses a three-dimensional ultrasonic diagnostic apparatus that makes it possible to reduce the cost of the apparatus and reduce power consumption by decreasing the number of delay circuits to be used for forming reception beams from reflection waves. An ultrasonic diagnostic apparatus of the present invention includes a three-dimensional ultrasonic scanning unit for carrying out a three-dimensional ultrasonic scanning by a plurality of ultrasonic transducers arranged two-dimensionally and for forming detection signals corresponding to reflection waves reflected from a subject that received waves applied by the transducers, a first delay add processing unit for dividing detection signal from all the transducers into a predetermined plurality of partial signal groups and for carrying out delay add processing to each of the divided partial signal groups to form a first delay adding signal, a second delay add processing unit for carrying out a delay add processing to the first delay adding signal output from the first delay add processing unit to form a second delay adding signal, and an image processing unit for reconstructing a three-dimensional image according to the second delay adding signal output from the second delay add processing unit. Based on this arrangement, it is possible to achieve a parallel simultaneous reception processing of a three-dimensional scanning with a small number of delay circuits. This facilitates a real-time display of the three-dimensional ultrasonic diagnostic apparatus.