1. An ultrasonic image generating device comprising: an ultrasonic probe which acquires a plurality of ultrasonic images obtained by scanning a subject from a plurality of directions with the ultrasonic probe; and a processing circuit which is configured to: obtain cross-section information indicating a position and an orientation of a specific cross-section of the subject; obtain positional information including a position and an orientation of each of the ultrasonic images of the subject; select at least one of the ultrasonic images originally obtained by the ultrasonic probe as a reference image, wherein the at least one of the ultrasonic images is separated from the specific cross-section by a distance that is less than a first threshold and has an orientation that is different from the orientation of the specific cross-section by less than a second threshold; and generate a cross-sectional image of the specific cross-section by calculating a pixel value of the cross-sectional image from an original pixel value of the reference image.