There is provided an ultrasound imaging apparatus that can reduce vertical stripes even though a transmission interval is widened for high speed imaging. When a receiving unit 1201 receives an instruction to perform a high speed imaging mode, an aperture synthesizing unit 1300 adds a predetermined number (N) of phase outputs obtained from the reflected ultrasonic waves of individual transmission beams at the same received focal points to reduce the occurrence of stripes on an image. In order to generate N phase outputs at each of the received focal points, a control unit 401 finds a necessary number (M) of the reception scanning lines to be set by the reception beam former 603, and notifies the reception beam former 603 of the number.
