1. An ultrasonic image generating device which generates a cross-sectional image of a specific cross-section of a subject from a plurality of ultrasonic images obtained by scanning the subject from a plurality of directions using an ultrasonic probe, the ultrasonic image generating device comprising: a cross-section position identifying unit configured to obtain cross-section information indicating a position and an orientation of the specific cross-section; a positional information obtaining unit configured to obtain positional information including a position and an orientation of each of the ultrasonic images of the subject; a reference image selecting unit configured to select at least one of the ultrasonic images as a reference image, the at least one of the ultrasonic images having a distance from the specific cross-section that is less than a first threshold and an orientation difference from the specific cross-section that is less than a second threshold; and a cross-sectional image generating unit configured generate the cross-sectional image using the reference image.