1. An ultrasonic diagnostic apparatus, comprising: an ultrasonic probe configured to generate an ultrasonic beam; a volume data set collector configured to collect a plurality of volume data sets corresponding to a plurality of three-dimensional scan ranges via the ultrasonic probe, the plurality of three-dimensional scan ranges partly overlapping one another; a region of interest setting device configured to set, in accordance with a user instruction, a region of interest on a first tomogram generated from a particular one of the plurality of volume data sets, the region of interest being set within a scan range of the plurality of three-dimensional scan ranges that corresponds to the particular one of the plurality of volume data sets; a tomogram generator configured to generate the first tomogram from the particular one of the volume data sets and also generate a second tomogram associated with a section including the region of interest from another volume data set of the plurality of volume data sets that has a corresponding three-dimensional scan range that includes the region of interest, wherein the tomogram generator specifies a position of the second tomogram in the another volume data set; and a display configured to display the first tomogram and the second tomogram.