Through the present invention, the amount of spatial variation of the amplification factor of a phased signal at each reception phasing point is reduced, and a high-quality image is obtained in aperture synthesis by an ultrasonic imaging device. In the present invention, an inter-transmission weight is generated which corresponds to a phasing range, the generated inter-transmission weight is applied to a reception phasing point in a phasing range obtained by transmission and reception in a reception beam former for performing aperture synthesis processing, and inter-transmission synthesis is performed. The inter-transmission weight smooths the variation of amplification factor between adjacent reception phasing points for each reception phasing point after inter-transmission synthesis, and is generated so that an amplification factor difference is reduced.