1. An ultrasonic diagnostic apparatus comprising: an ultrasonic probe configured to include a plurality of ultrasonic transducers; a transmission/reception unit configured to supply a driving signal to each of the ultrasonic transducers and generate a received signal corresponding to a scanned region based on each reception echo signal generated by each of the ultrasonic transducers; a B-mode data generation unit configured to generate three-dimensional B-mode data based on the received signal; a gain adjustment unit configured to execute gain adjustment for the three-dimensional B-mode data by using gain adjustment values respectively decided for a depth on each scanning line, an azimuth direction and an elevation direction in the scanned region; a threshold decision unit configured to decide a threshold used for discriminating a lumen region and a non-lumen region in the scanned region by using the three-dimensional B-mode data for which the gain adjustment has been executed; a threshold processing unit configured to execute threshold processing to discriminate data concerning the non-lumen region from the three-dimensional B-mode data for which the gain adjustment has been executed, by using the decided threshold; and an image generation unit configured to generate an ultrasonic image concerning the lumen region based on the three-dimensional B-mode data for which the threshold processing has been executed.